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CONVERSATIONS jnJ/2^ 


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ON 


STAWV&fUEo 




IN WHICH 


THE ELEMENTS 

OF THAT SCIENCE ARE FAMILIARLY EXPLAINED, 


AND ADAPTED TO 


THE COMPREHENSION OF YOUNG PUPILS 


Illustrated with Plates. 


XY THB AUTHOR OF CONVERSATIONS ON CHEMISTRY, AND CONVERSATIONS 

ON POLITICAL ECONOMY. 


PHILADELPHIA: 

PUBLISHED AND SOLD BY J. GRIGG, NO. 9 , NORTH FOURTH STREET; AND BY 

W. P. BASON, CHARLESTON, S. Cl 

1824, 







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I Crissy and G. Goodman, printers. 





PREFACE, 


It is with increased diffidence that the author 
offers this little work to the public. The encour¬ 
aging reception which the Conversations on Che¬ 
mistry and Political Economy have met with, 
has induced her to venture on publishing a short 
course on Natural Philosophy; but not without the 
greatest apprehensions for its success. Her igno¬ 
rance of mathematics, and the imperfect know¬ 
ledge of natural philosophy which that disadvan¬ 
tage necessarily implies, renders her fully sensible 
of her incompetency to treat the subject in any 
other way than in the form of a familiar expla¬ 
nation of the first elements, for the use of very 
young pupils. It is the hope of having done 
this in a manner that may engage their atten¬ 
tion, which encourages her to offer them these 
additional lessons. 


PREFACE. 


They are intended, in a course of elementary 
science, to precede the Conversations on Chemis¬ 
try; and were actually written previous to either 
of her former publications. 


CONTENTS. 



CONVERSATION I. 


rage. 

ON GENERAL PROPERTIES OP BODIES. 9 

Introduction. —General Properties of Bodies.—Impenetrability.— 
Extension.—Figure.—Divisibility.—Inertia.—Attraction.—Attrac¬ 
tion of Cohesion.—Density.—Rarity.—Heat.—Attraction of Gravi¬ 
tation. 


CONVERSATION II. 

ON THE ATTRACTION OP GRAVITY. 24 

Attraction of Gravitation, continued.—Of Weight.—Of the Fall of 
Bodies.—Of the resistance of the Air.—Of the Accent of Light 
Bodies. 


CONVERSATION HI. 

ON THE LAWS OF MOTION. 36 

Of Motion.—Of the Inertia of Bodies.—Of Force to Produce Motion. 
—Direction of Motion.—Velocity, absolute and relative.—Uniform 
Motion.—Retarded Motion.—Accelerated Motion.—Velocity of 
Falling Bodies.—Momentum.—Action and Reaction Equal.—Elas¬ 
ticity ot Bodies.—Porosity of Bodies.—Reflected Motion.—Angles 
of Incidence and Reflection. 

CONVERSATION IV. 

ON COMPOUND MOTION. 51 

Compound Motion, the result of two opposite forces.—Of Circular 
Motion, the result of two forces, one of which confines the body to 


VI 


CONTENTS 


a fixed point.—Centre of motion, the point at rest while the other 
parts of the body move round it.—Centre of Magnitude, the mid¬ 
dle of a body.—Centripetal force, that which confines a body to a 
fixed central point.—Centrifugal Force, that which impels a body 
to fly from the centre.—Fall of Bodies in a Parabola.—Centre of 
Gravity, the Centre of Weight, or point about which the parts ba¬ 
lance each other. 


CONVERSATION V\ 

OX THE MECHANICAL POWERS. 60 

Of the Power af Machines.—Of the Lever in general.—Of the Lever 
of the first kind, having the Fulcrum between the power and the 
weight,—Of the Lever of the second kind, having the Weight be¬ 
tween the power and the Fulcrum.—Of the Lever of the third 
kind, having the power between the Fulcrum and the weight.— 
Of the Pulley.—Of the Wheel and Axle.—Of the Inclined Plane. 
—Of the Wedge.—Of the screw. 

CONVERSATION VI. 

ASTRONOMY. 

CAUSES OF THE EARTHS ANNUAL MOTION. 79 

Of the Planets, and their motion.—Of the Diurnal motion of the Earth 
and Planets. 


CONVERSATION VII. 

ON THE PLANETS. 90 

Of the Satellites or Moons.—Gravity diminishes as the Square of the 
Distance.—Of the Solar System.—Of Comets.—Constellations, signs 
of the Zodiac.—Of Copernicus, Newton, &c. 

CONVERSATION VIII. 

ON THE EARTH. 102 

Of the Terrestrial Globe.—Of the Figure of the Earth.—Of the Pen¬ 
dulum.— Of the Variation of the Seasons, and of the Length of 
Days and Nights—Of the Causes of the Heat of Summer.—Of Solar, 
Siderial, and Equal or Mean Time. 


CONTENTS 


vii 


CONVERSATION IX. 

ON THE MOON. 121 

Of the Moon’s Motion.—Phases of the Moon.—Eclipses of tho Moon. 
—Eclipses of Jupiter’s Moons—Of the latitude and Longitude.— 
Of the Transits of the Inferior Planets.—Of the Tides. 

CONVERSATION X. 

HYDROSTATICS. 

ON THE MECHANICAL PROPERTIES OF FLUIDS. 132 

Definition of a Fluid.—Distinction between Fluids and Liquids.—Of 
Non-Elastic Fluids, scarcely susceptible of Compression.—Of the 
Cohesion of Fluids.—Of their Gravitation.—Of their Equilibrium. 
—Of their Pressure.—Of Specific Gravity.—Of the Specific Gravi¬ 
ty of Bodies heavier than Water.—Of those of the same weight as 
Water.—Of those lighter than Water.—Of the Specific Gravity of 
Fluids. 


CONVERSATION XI. 

OF SPRINGS, FOUNTAINS, &C. 144 

Of the Ascent of Vapour and the Formation of Clouds.—Of the For¬ 
mation and Fall of Rain, &c.—Of the Formation of Springs.—Of 
Rivers and Lakes.—Of Fountains. 

CONVERSATION XII. 


^ PNEUMATICS. 

ON THE MECHANICAL PROPERTIES OF AIR, 153 

Of the Spring or Elasticity of the Air.—Of the Weight of the Air.— 
Experiments with the Air Pump.—Of the Barometer.— Mode of 
Weighing Air.—Specific Gravity of Air. - Of Pumps.—Description 
of the Sucking Pump.—Description of the Forcing Pump. 

CONVERSATION XIII. 

ON WIND AND SOUND. 162 

Of Wind in General.—Of the Trade Wind.—Of the Periodical Trade 
Winds.—Of the Aerial Tides.—Of Sound in General.—Of Sonorous 


CONTENTS. 


via 

Bodies.—Of Musical Sounds.—Of Concord or Harmony, and Me¬ 
lody. 


CONVERSATION XIV. 

OK OPTICS. 1 77 

Of Luminous, Transparent, and Opaque Bodies.—Of the Radiation of 
Light.—Of Shadows.—Of the Reflection of Light.—Opaque Bo¬ 
dies seen only by Reflected Light.—Vision Explained.—Camera 
Obscura.—Image of Objects on the Retina. 

CONVERSATION XV. 


OK THE ANGLE OF VISION, AND REFLECTION OF MIRRORS. 190 

Angle of Vision.—Reflection of Plain Mirors.—Reflection of Convex 
Mirrors.—Reflection of Concave Mirrors. 

CONVERSATION XVI. 

OK REFRACTION AND COLOURS. 204 

Transmission of Light by Transparent Bodies.—Refraction.—Refrac¬ 
tion of the Atmosphere.—Refraction of a Lens.—Refraction of the 
Prism.—Of the Colours of Rays of Light.—Of the Colours of Bodies. 

CONVERSATION XVII. 

optics. 221 

OK THE STRUCTURE OF THE ETE, AND OPTICAL INSTRUMENTS 

Description of the Eye.—Of the Image on the Retina.—Refraction 
of the Humours of the Eye.—Of the use of Spectacles.—Of the 
Single Microscope.—Of the Double Microscope—Of the Solar 
Microscope.—Magic Lanthorn.—Refracting Telescope.—Reflect¬ 
ing Telescope. 


CONVERSATION 1. 


ON GENERAL PROPERTIES OF BODIES. 

INTRODUCTION.—GENERAL PROPERTIES OF BODIES.-IMPENETRABILITY, 

-EXTENSION.-FIGURE.-DIVISIBILITY.-INERTIA.-ATTRACTION.- 

ATTRACTION OF COHESION.-DENSITY.-RARITY.-HEAT,—ATTRACTION 

OF GRAVITATION. 


EMILY. 

I must request jour assistance, my dear Mrs. B. in a 
charge which I have latelv undertaken: it is that of in- 
structing my youngest sister, a task, w r hich I find proves 
more difficult than I had at first imagined. I can teach her 
the common routine of children’s lessons tolerably well; but 
she is such an inquisitive little creature, that she is not sa¬ 
tisfied without an explanation of every difficulty that occurs 
to her, and frequently asks me questions which I am at a 
loss to answer. This morning, for instance, when I had 
explained to her that the world was round like a ball, in¬ 
stead of being fiat as she had supposed, and that it was sur¬ 
rounded by the air, she asked me what supported it. I told 
her that it required no support; she then inquired why it 
did not fall as every thing else did? This I confess perplex¬ 
ed me; for I had myself been satisfied with learning that 
the world floated in the air, without considering how unna¬ 
tural it was that so heavy a body, bearing the weight of all 
other things, should be able to support itself. 

Mrs . B. I make no doubt, my dear, but that I shall be 
able to explain this difficulty to you; but I believe that it 
w r ould be almost impossible to render it intelligible to the 
comprehension of so young a child as your sister Sophia. 

2 


10 GENERAL PROPERTIES OF BODIES. 

You, who are now in your thirteenth year, may, I think, 
with great propriety, learn not only the cause of this parti¬ 
cular fact, but acquire a general knowledge of the laws by 
which the natural world is governed. 

Emily. Of all things, it is what I should most like to 
learn; but I was afraid it was too difficult a study even at 
my age. 

Mrs. B. Not when familiarly explained: if you have 
patience to attend, I will most willingly give you all the 
information in my power. You may perhaps find the sub¬ 
ject rather dry at first; but if I succeed in explaining the 
laws of nature, so as to make you understand them, I am 
sure that you will derive not only instruction, but great 
amusement from that study. 

Emily. I make no doubt of it, Mrs. B.; and pray begin 
by explaining why the earth requires no support; for that 
is the point which just now most strongly excites my cu¬ 
riosity. 

Mrs. B. My dear Emily, if I am to attempt to give you 
a general idea of the laws of nature, which is no less than to 
introduce you to a knowledge of the science of natural phi¬ 
losophy, it will be necessary for us to proceed with some 
degree of regularity. I do not wish to confine you to the 
systematic order of a scientific treatise, but if we were 
merely to examine every vague question that may chance to 
occur, our progress would be but very slow. Let us, there* 
fore, begin by taking a short survey of the general proper¬ 
ties of bodies, some of which must necessarily be explained 
before I can attempt to make you understand why the earth 
requires no support. 

When I speak of bodiesll mean substances, of whatever 
nature, whether solid or fluid; and matter is the general 
term used to denote the substance, whatever its nature be, 
of which the different bodies are composed. Thus, wood 
is the matter of which this table is made; water is the mat¬ 
ter with which this glass is filled, &c. 

Emily. I am very glad you have explained the meaning 
of the word matter, as it has corrected an erroneous con - 


GENERAL PROPERTIES OF BODIES. 11 

ception I had formed of it: I thought that it was applicable to 
solid bodies only. 

Mrs. B. There are certain properties which appear to 
be common to all bodies, and are hence called the essential 
properties of bodies; these are [Impenetrability, Extension , 
Figure , Divisibility, Inertia and Attraction. These are call¬ 
ed the general properties of bodies, as we do not suppose 
any body to exist without them. 

By impenetrability is meant the property which bodies 
have of occupying a certain space, so that, where one body 
is, another can not be, without displacing the former; for 
two bodies can not exist in the same place at the same time. 
A liquid may be more easily removed than a solid body; yet 
it is not the less substantial, since it is as impossible for a 
liquid and a solid to occupy the same space at the same 
time, as for two solid bodies to do so. For instance, if you 
put a spoon into a glass full of water, the water will flow 
over to make room for the spoon. 

- Emily. I understand this^perfectly. Liquids are in reality 
as substantial or as impenetrable as solid bodies, and they 
appear less so, only because they are more easily displaced. 

Mrs. B. The air is a fluid differing in its nature from li¬ 
quids, but no less impenetrable. If I endeavour to fill this 
phial by plunging it into this bason of water, the air, you 
see, rushes out of the phial in bubbles, in order to make 
way for the water, for the air and the water can not exist to¬ 
gether in the same space, any more than two hard bodies; 
and if I reverse this goblet, and plunge it perpendicularly 
into the water, so that the air will not be able to escape, 
the w’ater will no longer be able to fill the goblet. 

Emily. But it rises a considerable way into the glass. 

Mrs. B. Because the water compresses or squeezes the 
air into a small space in the upper part of the glass; but, as 
long as it remains there, no other body can occupy the same 
place. 

Emily. A difficulty has just occurred to me, with regard 
to the impenetrability of solid bodies; if a nail is driven in¬ 
to a piece of wood, it penetrates it, and both the wood and 
the nail occupy the same space that the wood alone did be¬ 
fore ? 


\% GENERAL PROPERTIES OF BODIES. 

Mrs. B. The nail penetrates between the particles of 
the wood, by forcing them to make way for it; for you know 
that not a single atom of wood can remain in the space 
which the nail occupies; and if the wood is not increased 
in size by the addition of the nail, it is because wood is a 
porous substance, like sponge, the particles of which may 
be compressed or squeezed closer together; and it is thus 
that they make way for the nail. 

We may now proceed to the next general property of bo¬ 
dies, extension. A body which occupies a certain space 
must necessarily have extension; that is to say, length , breadth 
and depth; these are called the dimensions of extension: can 
you form an idea of any body without them? 

Emily. No; certainly I can not; though these dimensions 
must, of course vary extremely in different bodies. The 
length, breadth and depth of a box, or of a thimble, are very 
different from those of a walking stick, or of a hair. 

But is not height also a dimension of extension? 

Mrs. B. Height and depth are the same dimension, con¬ 
sidered in different points of view; if you measure a body, 
or a space, from the top to the bottom, you call it depth; if 
from the bottom upwards, you call it height; thus the depth 
and height of a box are, in fae*, the same thing. 

Emily. Very true; a moment’s consideration would have 
enabled me to discover that; and breadth and width are al¬ 
so the same dimension. 

Mrs. B. Yes; the limits of extension constitute figure 
or shape. You conceive that a body having length, breadth 
and depth, can not be without form, either symmetrical or 
irregular? 

Emily. Undoubtedly; and this property admits of almost 
an infinite variety. 

Mrs. B. Nature has assigned regular forms to her pro¬ 
ductions in general. The natural form of mineral substances 
is that of chrystals, of which there is a great variety. Many 
of them are very beautiful, and no less remarkable by their 
transparency or colour, than by the perfect regularity of their 
forms, as maybe seen in the various museums and collections 


GENES A-'. .’R- BODIES. IS 

of natural history. Tb* . table *\«d animal creation ap¬ 
pears less symmetric ; is sttli more diversified in figure 
than the mineral kingdiu Manufactured substances as¬ 
sume the various ar tra forms which the art of man de¬ 
signs for them; and m infinite number of irregular forms 
are produced by fractureand by the dismemberment of 
the parts of bodies. 

Emily. Such as a piece of broken china, or glass? 

Mrs. B. Or the fragments of mineral bodies which are 
broken in being dug out of the earth, or decayed by the ef¬ 
fect of torrents and other causes. The picturesque effect of 
rock-scenery is in a gioat measure owing to accidental irre¬ 
gularities of this kind. 

We may now proceed to divisibility; that is to say, a sus¬ 
ceptibility of being divided into an indefinite number of 
parts. Take any small quantity of matter, a grain of sand for 
instance, and cut it into two parts; these two parts might 
be again divided, had we instruments sufficiently fine for the 
purpose; and if by means of pounding, grinding-, and other 
similar methods, we carry this division to the greatest pos¬ 
sible extent, and reduce the body to its finest imaginable par¬ 
ticles, yet not one of the particles will be destroyed, and the 
body will continue to exist, though in this altered state. 

The melting of a solid body in a liquid affords a very 
striking example of the extreme divisibility of matter; when 
you sweeten a cup of tea, for instance, with what minute¬ 
ness the sugar must be divided to be diffused throughout the 
whole of the liquid. 

Emily. And if you pour a few drops of red wine into a 
glass of water, they immediately tinge the whole of the wa¬ 
ter, and must therefore be diffused throughout it. 

Mrs. B. Exactly so; and the perfume of this lavender 
water will be almost as instantaneously diffused thoughout 
the room, if I take out the stopper. 

Emily. But in this case it is only the perfume of the laven¬ 
der, and not the water itself that i^^ffused in the room? 

Mrs. B. The odour or smi|^pPoody is part of the body 
itself, and is produced by veryromute particles or exhala¬ 
tions which escape from odoriferous bodies. It would be 
2 * 


14 


GENERAL PROPERTIES OF BODIES. 


impossible that you should smell the lavender water, if 
particles of it did not come in actual contact with your nose. 

Emily. But when I smell a flower, I see no vapour rise 
from it; and yet I perceive the smell at a considerable dis¬ 
tance. 

Mrs. B. You could, I assure you, no more smell a flower, 
the odoriferous particles of which did not touch your nose, 
than you could taste a fruit, the flavoured particles of which 
did not come in contact with your tongue. 

Emily. That is wonderful indeed; the particles then, 
which exhale from the flower and from the lavender water, 
are, I suppose, too small to be visible? 

Mrs. B. Certainly: you may form some idea of their ex¬ 
treme minuteness, from the immense number which must 
have escaped in order to perfume the whole room; and yet 
there is no sensible diminution of the liquid in the phial. 

Emily. But the quantity must really be diminished? 

Mrs. B. Undoubtedly; and were you to leave the bottle 
open a sufficient length of time, the whole of the water 
would evaporate and disappear. But though so minutely 
subdivided as to be imperceptible to any of our senses, each 
particle would continue to exist; for it is not within the pow¬ 
er of man to destroy a single particle of matter: nor is there 
any reason to suppose that in nature an atom is ever anni¬ 
hilated. 

Emily. Yet, when a body is burnt to ashes, part of it, at 
least, appears to be effectually destroyed? Look how small 
is the residue of ashes beneath the grate, from all the coals 
which have been consumed within it. 

Mrs. B. That part of the coals, which you suppose to 
be destroyed, evaporates in the form of smoke and vapour, 
whilst the remainder is reduced to ashes. A body, in burning, 
undergoes no doubt very remarkable changes; it is-generally 
subdivided; its form and colour altered; its extension increas¬ 
ed: but the various parts, into which it has been separated 
by combustion, continue in existence, and retain all the es¬ 
sential properties of bodies.. 

Emily. But that part oWburnt body which evaporates 
in smoke has no figure; smoke, it is true, ascends in columns 



GENERAL PROPERTIES OF BODIES. 15 

into the air, but it is soon so much diffused as to lose all 
form; it becomes indeed invisible. 

Mrs. B. Invisible, I allow; but we must not imagine 
that what we no longer see no longer exists. Were every 
particle of matter that becomes invisible annihilated, the 
world itself would in the course of time be destroyed. The 
particles of smoke, when diffused in the air, continue still to 
be particles of matter as well as when more closely united 
in the form of coals: they are really as substantial in the one 
state as in the other, and equally so when by their extreme 
subdivision they become invisible. No particle of matter is 
ever destroyed: this is a principle you must constantly re¬ 
member. Every thing in nature decays and corrupts in the 
lapse of time. We die, and our bodies moulder to dust; but 
not a single atom of them is lost; they serve to nourish the 
earth, whence, while living, they drew their support. 

The next essential property of matter is called inertia; 
this word expresses the resistance which inactive matter 
makes to a change of state. Bodies appear to be equally in¬ 
capable of changing their actual state, whether it be of mo¬ 
tion or of rest. You know that it requires force to put a body 
which is at rest in motion; an exertion of strength is also re¬ 
quisite to stop a body which is already in motion. The re¬ 
sistance of the body to a change of state, in either case, is 
called its inertia. 

Emily. In playing at base-ball I am obliged to use all 
my strength to give a rapid motion to the ball; and when I 
have to catch it, I am sure I feel the resistance it makes to 
being stopped. But if I did not catch it, it would soon fall 
to the ground and stop of itself. 

Mrs. B. Inert matter is as incapable of stopping of itself 
as it is of putting itself into motion: when the ball ceases to 
move, therefore, it must be stopped by some other cause or 
power; but as it is one with which you are yet unacquaint¬ 
ed, we can not at present investigate its effects. 

The last property which appears to be common to all 
bodies is attraction. All bodies consist of infinitely small 
particles of matter, each of which possesses the power of at¬ 
tracting or drawing towards it, and uniting with any other 



16 


GENERAL PROPERTIES OF BODIES. 


particle sufficiently near to be within the influence of ^at¬ 
traction; but in minute particles this power extends to so’ 
very small a distance around them, that its effect is not sen¬ 
sible, unless they are (or at least appear to be) in contact; 
it then makes them stick or adhere together, and is hence 
called the attraction of cohesion. Without this power, solid 
bodies would fall in pieces, or rather crumble to atoms. 

Emily. I am so much accustomed to see bodies firm and 
solid, that it never occurred to me that any power was re¬ 
quisite to unite the particles of which they are composed. 
But the attraction of cohesion does not, I suppose, exist in 
liquids; for the particles of liquids do not remain together 
so as to form a body, unless confined in a vessel? 

Mrs. B. I beg your pardon; it is the attraction of cohe¬ 
sion which holds this drop of water suspended at the end of 
my finger, and keeps the minute watery particles of which 
it is composed united. But as this power is stronger in pro¬ 
portion as the particles of bodies are more closely united, 
the cohesive attraction of solid bodies is much greater than 
that of fluids. 

The thinner and lighter a fluid is, the less is the cohe¬ 
sive attraction of its particles, because they are further apart; 
and in elastic fluids, such as air, there is no cohesive attrac¬ 
tion among the particles. 

Emily . That is very fortunate; for it would be impossi¬ 
ble to breathe the air in a solid mass; or even in a liquid 
state. 

But is the air a body of the same nature as other bodies? 

Mrs. B. Undoubtedly, in all essential properties. 

Emily. Yet you say that it does not possess one of the 
general properties of bodies—cohesive attraction? 

Mrs. B. The particles of air are not destitute of the 
power of attraction, but they are too far distant from each 
other to be influenced by it: and the utmost efforts of human 
art have proved ineffectual in the attempt to compress 
them, so as to bring them within the sphere of each other’s 
attraction, and make them cohere. 

Emily. If so, how is it possible to proYe that they are 
endowed with this power? 


/ 



GENERAL PROPERTIES OF BODIES. 


17 


^ Mrs. )j. The air is formed of particles precisely of the 
same nature as those which enter into the composition of 
liquid and solid bodies, in which state we have a proof of 
their attraction. 

Emily. It is then, I suppose, owing to the different de¬ 
grees of attraction of different substances, that they are hard 
or soft; and that liquids are thick or thin. 

Mrs. B. Yes; but you would express your meaning bet¬ 
ter by the term density, which denotes the degree of close¬ 
ness and compactness of the particles of a body: thus you may 
say, both of solids, and of liquids, that the stronger the co¬ 
hesive attraction, the greater is the density of the body. In 
philosophical language, density is said to be that property of 
bodies by which they contain a certain quantity of matter, 
under a certain bulk or magnitude. Rarity is the contrary 
of density; it denotes the thinness and subtlety of bodies: 
thus you would say that mercury or quicksilver was a very 
dense fluid; ether, a very rare one, &c. 

Caroline. But how are we to judge of the quantity of 
matter contained in a certain bulk? 

Mrs. B. By the weight: under the same bulk bodies are 
said to be dense in proportion as they are heavy. 

Emily. Then we may say that metals are dense bodies, 
wood comparatively a rare one, &c. But, Mrs. B., when 
the particles of a body are so near as to attract each other, 
the effect of this power must increase as they are brought 
by it closer together; so that one would suppose that the body 
would gradually augment in density, till it was impossible 
for its particles to be more closely united. Now, we know 
that this is not the case; for soft bodies, such as cork, sponge, 
or butter, never become, in consequence of the increasing 
attraction of their particles, as hard as iron? 

Mrs. B. In such bodies as cork and sponge, the parti¬ 
cles which come in contact are so few as to produce but a 
slight degree of cohesion: they are porous bodies, which, 
owing to the peculiar arrangement of their particles, abound 
with interstices which separate the particles; and these va¬ 
cancies are filled with air, the spring or elasticity of which 
prevents the closer union of the parts. Bi*t there is another 



IS 


GENERAL PROPERTIES OF BODIES. 


fluid much more subtle than air, which pervades all bodies, 
this is heat. Heat insinuates itself more or less between the 
particles of all bodies, and forces them asunder; you may 
therefore consider heat, and the attraction of cohesion, as 
constantly acting in opposition to each other. 

Emily. The one endeavouring to rend a body to pieces, 
the other to keep its parts firmly united. 

Mrs. B. And it is this struggle between the contending 
forces of heat and attraction, which prevents the extreme 
degree of density which would result from the sole influence 
of the attraction of cohesion. 

Emily. The more a body is heated then, the more its 
particles will be separated. 

Mrs. B. Certainly: we find that bodies swell or dilate 
by heat: this effect is very sensible in butter, for instance, 
which expands by the application of heat, till at length the 
attraction of cohesion is so far diminished that the particles 
separate, and the butter becomes liquid. A similar effect 
is produced by heat on metals, and all bodies susceptible of 
being melted. Liquids, you know, are made to boil by the 
application of heat; the attraction of cohesion then yields 
entirely to the expansive power; the particles are totally 
separated and converted into steam or vapour. But the 
agency of heat is in no body more sensible than in air, 
which dilates and contracts by its increase or diminution in 
a very remarkable degree. 

Emily . The effects of heat appear to be one of the most 
interesting parts of natural philosophy. 

Mrs , B. That is true; but heat is so intimately con¬ 
nected with chemistry, that you must allow me to defer the 
investigation of its properties till you become acquainted 
with that science. 

To return to its antagonist, the attraction of cohesion; it 
is this power which restores to vapour its liquid form, which 
unites it into drops when it falls to earth in a shower of 
rain, which gathers the dew into brilliant gems on the 
blades of grass. 

Emily. And I have often observed that after a shower, 
the water collects into large drops on the leayes of plants; 


GENERAL PROPERTIES OF BODIES. 19 

but I can not say that I perfectly understand how the at¬ 
traction of cohesion produces this effect. 

Mrs. B. Rain does not fall from the clouds in the form 
of drops, but in that of mist or vapour, which is composed 
of very small watery particles; these in their descent mu¬ 
tually attract each other, and those that are sufficiently near 
in consequence unite and form a drop, and thus the mist is 
transformed into a shower. The dew also was originally 
in a state of vapour, but is, by the mutual attraction of the 
particles, formed into small globules on the blades of grass: 
in a similar manner the rain upon the leaf collects into large 
drops, which when they become too heavy for the leaf to 
support, fall to the ground. 

Emily . All this is wonderfully curious! I am almost be¬ 
wildered with surprise and admiration at the number of new 
ideas I have already acquired. 

Mrs. B. Every step that you advance in the pursuit of 
natural science, will fill your mind with admiration and 
gratitude towards its Divine Author. In the study of natu¬ 
ral philosophy, we must consider ourselves as reading the 
book of nature, in which the bountiful goodness and wisdom 
of God is revealed to all mankind; no study can then tend 
more to purify the heart, and raise it to a religious contem¬ 
plation of the Divine perfections. 

There is another curious effect of the attraction of cohe¬ 
sion which I must point out to you. It enables liquids to 
rise above their level in capillary tubes: these are tubes, the 
bores of which are so extremely small that liquids ascend 
within them, from the cohesive attraction between the parti¬ 
cles of the liquid and the interior surface of the tube. Do you 
perceive the water rising above its level in this small glass 
tube, which I have immersed in a goblet full of water? 

Emily . Oh yes; I see it slowly creeping up the tube, but 
now it is stationary: will it rise no higher? 

Mrs, B. No; because the cohesive attraction between 
the water and the internal surface of the tube is now balan¬ 
ced by the weight of the water within it; if the bore of the 
tube were narrower the water would rise higher; and if you 
immerse several tubes of bores of different sizes, you will 


20 


GENERAL PROPERTIES OF CODIES. 


see it rise to different heights in each of them. In making 
this experiment, you should colour the water with a little red 
wine, in order to render the effect more obvious. 

All porous substances, such as sponge, bread, linen, &c. 
maybe considered as collections of capillary tubes: if you 
clip one end of a lump of sugar into water, the w 7 ater will 
rise in it, and wet it considerably above the surface of that 
into which you dip it. 

Emily. In making tea I have often observed that effect, 
without being able to account for it. 

Mrs. B. Now that you are acquainted with the attrac¬ 
tion of cohesion, I must endeavour to explain to you that of 
Gravitation , which is a modification of the same power; the 
first is perceptible only in very minute particles, and at ve¬ 
ry small distances; the other acts on the largest bodies, and 
extends to immense distances. 

Emily. You astonish me: surely you do not mean to say 
that large bodies attract each other? 

Mrs. B. Indeed I do: let us take, for example, one of 
the largest bodies in nature, and observe whether it does not 
attract other bodies. What is it that occasions the fall of 
this book, when I no longer support it? 

Emily. Can it be the attraction of the earth? I thought 
that all bodies had a natural tendency to fall. 

Mrs. B. They have a natural tendency to fall, it is true; 
but that tendency is produced entirely by the attraction of 
the earth: the earth being so much larger than any body on 
its surface, forces every body, which is not supported, to fall 
upon it. 

Emily. If the tendency which bodies have to fall results 
from the earth’s attractive power, the earth itself ean have 
no such tendency, since it can not attract itself, and there¬ 
fore it requires no support to prevent it from falling. Yet 
the idea that bodies do not fall of their own accord, but that 
they are drawn towards the earth by its attraction, is so new 
and strange to me, that I know not how to reconcile my¬ 
self to it. 

Mrs. B. When you are accustomed to consider the fall 
of bodies as depending on this cause, it will appear to you 


GENERAL PROPERTIES OP BODIES. 


21 


as natural, and surely much more satisfactory, than if the 
cause of their tendency to fall were totally unknown. Thus 
you understand that all matter is attractive, from the small¬ 
est particle to the largest mass; and that bodies attract each 
other with a force proportional to the quantity of matter they 
contain. 

Emily#* I do not perceive any difference between the at¬ 
traction of cohesion and that of gravitation; is it not because 
every particle of matter is endowed with an attractive pow¬ 
er, that large bodies consisting of a great number of parti¬ 
cles, are so strongly attractive? 

Mrs. B. True. There is, however, this difference be¬ 
tween the attraction of particles and that of masses, that 
the former is stronger than the latter, in proportion to the 
quantity of matter. Of this you have an instance in the at¬ 
traction of capillary tubes, in which liquids ascend by the 
attraction of cohesion, in opposition to that of gravity. It 
is on this account that it is necessary that the bore of the 
tube should be extremely small; for if the column of water 
within the tube is not very minute, the attraction would not 
be able either to raise or support its weight, in opposition 
to that of gravity. 

You may observe also, that all solid bodies are enabled 
by the force of the cohesive attraction of their particles to 
resist that of gravity, which would otherwise disunite them, 
and bring them to a level with the ground, as it does in the 
case of liquids, the cohesive attraction of which is not suf¬ 
ficient to enable it to resist the power of gravity. . 

Emily. And some solid bodies appear to be of this na¬ 
ture,. as sand and powder for instance: there is no attrac¬ 
tion of cohesion between their particles? 

Mrs. B. Every grain of powder or sand is composed of 
a great number of other more minute particles, firmly uni¬ 
ted by the attraction of cohesion; but'amongst the separate 
grains there is no sensible attraction, because they are not 
in sufficiently close contact. 

Emily. Yet they actually touch each other? 

Mrs. B. The surface of bodies is in general so rough 

3 


22 GENERAL PROPERTIES OF BODIES. 

and uneven, that when in actual contact, they touch each 
other only by a few points. Thus, if I lay upon the table 
this book, the binding of which appears perfectly smooth, 
yet so few of the particles of its under surface come in con¬ 
tact with the table, that no sensible degree of cohesive at¬ 
traction takes place; for you see that it does not stick or 
cohere to the table, and I find no difficulty in lifting it off. 

It is only when surfaces, perfectly flat and well polish¬ 
ed, are placed in contact, that the particles approach in suf¬ 
ficient number, and closely enough, to produce a sensible 
degree of cohesive attraction. Here are two hemispheres 
of polished metal, I press their flat surfaces together, hav¬ 
ing previously interposed a few drops of oil, to fill up every 
little porous vacancy. Now try to separate them. 

Emily. It requires an effort beyond my strength, though 
there are bandies for the purpose of pulling them asunder. 
Is the firm adhesion of the two hemispheres merely owing 
to the attraction of cohesion? 

Mrs. B. There is no force more powerful, since it is 
by this that the particles of the hardest bodies are held to¬ 
gether. If would require a weight of several pounds to 
separate these hemispheres. 

Emily. In making a kaleidoscope, I recollect that the 
two plates of glass, which were to serve as mirrors, stuck 
so fast together, that I imagined some of the gum I had been 
using had by chance been interposed between them; but 
now I make no doubt but that it was their own natural cohe¬ 
sive attraction which produced this effect. 

Mrs. B. Very probably it was so; for plate-glass has an 
extremely smooth, flat surface, admitting of the contact of 
a great number of particles, between two plates, laid one 
over the other. 

Emily. But, Mrs. B., the cohesive attraction of some 
is much greater than that of others; thus glue, gum and 
paste, cohere with singular tenacity. 

Mrs . B. That is owing to the peculiar chemical pro¬ 
perties of those bodies, independently of their cohesive at¬ 
traction. 


GENERAL PROPERTIES OF BODIES. 


23 


There are some other kinds of modifications of attrac¬ 
tion peculiar to certain bodies; namely, that of magnetism, 
and of electricity; but we shall confine our attention mere¬ 
ly to the attraction of cohesion and of gravity; the exami¬ 
nation of the latter we shall resume at our next meeting. 


CONVERSATION II 


ON THE ATTRACTION OP GRAVITY. 


ATTRACTION OF GRAVITATION, CONTINUED.—OF WEIGHT.-OF THE FALL- 

CF BODIES.-OF THE RESISTANCE OF THE AlR,—OF THE ASCENT OF LIGHT 

BODIES. 

i 

EMILY. 

I have related to my sister Caroline all that you iiave 
taught me of natural philosophy, and she has been so much 
delighted by it, that she hopes you will have the goodness 
to admit her to your lessons. 

Mrs B. Very willingly; but I did not think you had any 
taste for studies of this nature, Caroline? 

Caroline. I confess, Mrs. B., that hitherto I had formed 
no very agreeable idea either of philosophy, or philosophers; 
but what Emily has told me has excited my curiosity so 
much, that I shall be highly pleased if you will allow me 
to become one of your pupils. 

Mrs. B. I fear that I shall not find you so tractable a 
scholar as Emily; I know that you are much biased in fa¬ 
vour of your own opinions. 

Caroline. Then you will have the greater merit in re¬ 
forming them, Mrs. B.; and after all the wonders that Emi¬ 
ly has related to me, I think I stand but little chance against 
you and your attractions. 

Mrs. B. You will, I doubt not, advance a number of 
objections; but these I shall willingly admit, as they will be 
a means of elucidating the subject. Emily, do you recol¬ 
lect the names of the general properties of bodies? 


ON THE ATTRACTION OF GRAVITY. 26 

Emily. Impenetrability, extension, figure, divisibility, 
inertia and attraction. 

Mrs. B. Very well. You must remember that these 
are properties common to all bodies, and of which they can 
not be deprived; all other properties of bodies are called 
accidental, because they depend on the relation or connec¬ 
tion of one body to another. 

Caroline. Yet surely, Mrs. B. there are other properties 
which are essential to bodies, besides those you have enu¬ 
merated. Colour and weight, for instance, are common to 
all bodies, and do not arise from their connection with each 
other, but exist in the bodies themselves; these, therefore, 
can not be accidental qualities? 

Mrs. B. I beg your pardon; these properties do not ex¬ 
ist in bodies independently of their connection with other 
bodies. 

Caroline. What! have bodies no weight? Does not this 
table weigh heavier than this book; and, if one thing weighs 
heavier than another, must there not be such a thing as 
weight? 

Mrs. B. No doubt: but this property does not appear 
to be essential to bodies; it depends upon their connection 
with each other. Weight is an effect of the power of at¬ 
traction, without which the table and the book would have 
no weight whatever. 

Emily. I think I understand you; is it not the attraction 
of gravity which makes bodies heavy? 

Mrs. B. You are right. I told you that the attraction 
of gravity was proportioned to the quantity of matter which 
bodies contained: now the earth consisting of a much great¬ 
er quantity of matter than any body upon its surface, the 
force of its attraction must necessarily be greatest, and must 
draw every thing towards it; in consequence of which, bo¬ 
dies that are unsupported fall to the ground, whilst those 
that are supported press upon the object which prevents 
their fall, with a weight equal to the force with which they 
gravitate towards the earth. 

Caroline. The same cause then which occasions the fall 
of bodies, produces also their weight. It was verv dull in 

3 * 


26 


ON THE ATTRACTION OP GRAVITY. 


me not to understand this before, as it is the natural and 
necessary consequence of attraction; but the idea that bo* 
dies were not really heavy of themselves, appeared to me 
quite .incomprehensible. But, Mrs. B. if attraction is a pro¬ 
perty essential to matter, weight must be so likewise; for 
liow can one exist without the other? 

Mrs. B. Suppose there were but one body existing in 
universal space, what would its weight be? 

Caroline. That would depend upon its size; or more 
accurately speaking, upon the quantity of matter it con¬ 
tained. 

Emily. No, no; the body would have no weight, what¬ 
ever were its size; because nothing would attract it. Am 
I not right, Mrs. B.? 

Mrs. B. You are: you must allow, therefore, thauit 
would be possible for attraction to exist without weight; 
for each of the particles of which the body was composed, 
would possess the power of attraction; but they could exert 
it only amongst themselves; the whole mass having nothing 
to attract, or to be attracted by, would have no weight. 

Caroline. I am now well satisfied that weight is not 
essential to the existence of bodies; but what have you to 
object to colours, Mrs. B.; you will not, I think, deny that 
they really exist in the bodies themselves. 

Mrs. B. When we come to treat of the subject of co¬ 
lours, I trust that I shall be able to convince you, that co¬ 
lours are likewise accidental qualities, quite distinct from 
the bodies to which they appear to belong. 

Caroline. Oh do pray explain it to us now, I am so very 
curious to know how that is possible. 

Mrs. B. Unless w r e proceed with some degree of order 
and method, you will in the end find yourself but little the 
wiser for all you learn. Let us therefore go on regularly, 
and make ourselves well acquainted with the general pro¬ 
perties of bodies before we proceed further. 

Emily. To return, then, to attraction, (which appears 
to me by far the most interesting of them, since it belongs 
equally to all kinds of matter) it must be mutual between 


ON THE ATTRACTION OF GRAVITY. 27 

two bodies; and if so, when a stone falls to the earth, the 
earth should rise part of the way to meet the stone? 

Mrs. B. Certainly; but you must recollect that the 
force of attraction is proportioned to the quantity of matter 
which bodies contain, and if you consider the difference 
there is in that respect, between a stone and the earth, you 
will uot be surprised that you do not perceive the earth rise 
to meet the stone; for though it is true that a mutual attrac¬ 
tion takes place between the earth and the stone, that of 
the latter is so very small in comparison to that of the for¬ 
mer, as to render its effect insensible. 

Emily. But since attraction is proportioned to the quan¬ 
tity of matter which bodies contain, why do not the hills 
attract the houses and churches towards them? 

Caroline. Heavens, Emily, what an idea! How can the 
houses and churches be moved, when they are so firmly 
fixed in the ground! 

Mrs. B. Emily’s question is not absurd, and your an¬ 
swer, Caroline, is perfectly just; but can you tell us why 
the houses and churches are so firmly fixed in the ground? 

Caroline. I am afraid I have answered right by mere 
chance; , for I begin to suspect that bricklayers and carpen¬ 
ters could give but little stability to their buildings, without 
the aid of attraction. 

Mrs. B. It is certainly the cohesive attraction between 
the bricks and the mortar, which enables them to build 
walls, and these are so strongly attracted by the earth, as to 
resist every other impulse; otherwise they would necessa¬ 
rily move towards the hills and the mountains; but the less¬ 
er force must yield to the greater. There are, however, 
some, circumstances in which the attraction of a large body 
has sensibly counteracted that of the earth. If ivhilst stand¬ 
ing on the declivity of a mountain, you hold a plumb-line 
in your hand, the weight will not fall perpendicular to the 
earth, but incline a little towards the mountain; and this is 
owing to the lateral, or sideways attraction of the mountain, 
interfering with the perpendicular attraction of the earth. 

Emily. But the size of a mountain is very trifling, com¬ 
pared to the whole earth. 


2S 


ON THE ATTRACTION OF GRAVITY. 


Mrs. B. Attraction, you must recollect, diminishes with 
distance; and in the example of the plumb-line, the weight 
suspended is considerably nearer to the mountain than to 
the centre of the earth. 

Caroline. Pray Mrs. B. do the two scales of a balance 
hang parallel to each other? 

Mrs. B. You mean, I suppose, in other words to inquire 
whether two lines which are perpendicular to the earth, are 
parallel to each other? I believe I guess the reason of your 
question; but I wish you would endeavour to answer it 
without my assistance. 

Caroline. I w r as thinking that such lines must bolh tend 
by gravity to the same point, the centre of the earth; now 
lines tending to the same point can not be parallel, as paral¬ 
lel lines are always at an equal distance from each mother, 
and would never meet. 

Mrs. B. Very well explained; you see now the use of 
your knowledge of parallel lines: had you been ignorant of 
their properties, you could not have drawn such a conclu¬ 
sion. This may enable you to form an idea of the great 
advantage to be derived even from a slight knowledge of 
geometry, in the study of natural philosophy; and if, after 
I have made you acquainted with the first elements, you 
should be tempted to pursue the study, I would advise you 
to prepare yourselves by acquiring some knowledge of ge¬ 
ometry. This science would teach you that lines which 
fall perpendicular to the surface of a sphere can not be 
parallel, because they would .all meet, if prolonged to the 
centre of the sphere; while lines that fall perpendicular to 
a plane or flat surface, are always parallel, because if pro¬ 
longed, they would never meet. 

Emily. And yet a pair of scales, hanging perpendicular 
to the earth, appear parallel? 

Mrs. B. Because the sphere is so large, and the scales 
consequently converge so little, that their inclination is not 
perceptible to our senses; if we could construct a pair of 
scales whose beam would extend several degrees, their con¬ 
vergence would be very obvious; but as this can not be ac¬ 
complished, let us draw a small figure of the earth, and 
























































































ON THE ATTRACTION OF GRAVITY. 29 

then we may make a pair of scales of the proportion we 
please, (fig. 1. pi. I.) 

Caroline. This figure renders it very clear: then two 
bodies can not fall to the earth in parallel lines? 

Mrs. B. Never. 

Caroline. The reason that a heavy body falls quicker 
than a light one, is, I suppose, because the earth attracts it 
more strongly? 

Mrs. B. The earth, it is true, attracts a heavy body 
more than a light one; but that would not make the one 
fall quicker than the other. 

Caroline. Yet. since it is attraction that occasions the 
fait cf Uudies, surely the more a body is attracted, the more 
rapidly it will fall. Besides, experience proves it to be so. 
Do we not every day see heavy bodies fall quickly, and 
light bodies slowly. 

Emily. It strikes me, as it does Caroline, that as attrac¬ 
tion is proportioned to the quantity of matter, the earth must 
necessarily attract a body which contains a great quantity 
more strongly, and therefore bring it to the ground sooner 
than one consisting of a smaller quantity. 

Mrs. B. You must consider, that if heavy bodies are at¬ 
tracted more strongly than light ones, they require more at¬ 
traction to make them fall. Remember that bodies have 
no natural tendency to fall, any more than to rise, or to 
move laterally, and that they will not fall unless impelled 
by some force; now this force must be proportioned to the 
quantity of matter it has to move: a body consisting of 1000 
particles of matter, for instance, requires ten times as much 
attraction to bring it to the ground in the same space of 
time as a body consisting of only 100 particles. 

Caroline. I do not understand that; for it seems to me, 
that the heavier a body is, the more easily and readily it 
faJIs. 

Emily. I think I now comprehend it; let me try if I can 
explain it to Caroline. Suppose that I draw towards me 
two weighty bodies, the one of lOOlbs. the other of lOOOlbs. 
must I not exert ten times as much strength to draw the 
larger one to me, in the same space of time as is required 


30 


ON THE ATTRACTION OF GRAVITY. 


for the smaller one? And if the earth draws a body of 
lOOOlbs. weight to it in the same space of time that it draws 
a body of lOOlbs. does it not follow that it attracts the body 
of lOOOlbs. weight with ten times the force that it does that 
of lOOlbs.? 

Caroline. I comprehend your reasoning perfectly; but if 
it were so, the body of lOOOlbs. weight, and that of lOOlbs. 
would fall with the same rapidity; and the consequence 
would be, that all bodies, whether light or heavy, being at 
an equal distance from the ground, would fall to it in the 
same space of time: now it is very evident that this conclu¬ 
sion is absurd; experience every instant contradicts it; ob¬ 
serve how much sooner this book reaches the Hoo* than this 
sheet of paper, when I let them drop together. 

Emily. That is an objection I can not answer. T must 
refer it to you, Mrs. B. 

JMrs. B. I trust that w£ shall not find it insurmountable. 
It is true that, according to the laws of attraction, all bodies 
at an equal distance from the earth, should fall to it in the 
same space of time; and this would actually take place if 
no obstacle intervened to impede their fall. But bodies fall 
through the air, and it is the resistance of the air which 
makes bodies of different density fall with different degrees 
of velocity. They must all force their way through the air, 
but dense heavy bodies overcome this obstacle more easily 
than rarer or lighter ones. 

The resistance which the air opposes to the fall of bodies 
is proportioned to their surface, not to their weight; the air 
being inert, can not exert a greater force to support the 
weight of a cannon ball, than it does to support the weight 
of a ball (of the same size) made of leather; but the cannon 
ball will overcome this resistance more easily, and fall to 
the ground, consequently, quicker than the leather ball. 

Caroline. This is very clear and solves the difficulty 
perfectly. The air offers the same resistance to a bit of 
lead and a bit of feather of the same size; yet the one 
seems to meet with no obstruction in its fall, whilst the 
other is evidently resisted and supported for some time by 
the air. 


ON THE ATTRACTION OF GRAVITY. 


31 


Emily The larger the surface of a body, then, the more 
air it covers, and the greater is the resistance it meets with 
from it. 

Mrs. B. Certainly: observe the manner in which this 
sheet of paper falls; it floats awhile in the air, and then 
gently descends to the ground I will roll the same piece 
of paper up into a ball: it offers now but a small surface to 
the air, and encounters therefore but little resistance: see 
'how much more rapidjy it falls. 

The heaviest bodies may be made to float awhile in the 
air, by making the extent of their surface counterbalance 
their weight. Here is some gold, which is the most dense 
body we are acquainted with, but it has been beaten into a 
very thin leaf, and offers so great an extent of surface in 
proportion to its weight, that its fall, you see, is still more 
retarded by the resistance of the air than that of the sheet 
of paper. 

Caroline. That is very curious: and it is, I suppose, 
upon the same principle that iron boats may be made to 
float on water? 

But, Mrs. B., if the air is a real body, is it not also sub¬ 
jected to the laws of gravity? 

Mrs. B. Undoubtedly. 

Caroline. Then why does it not, like all other bodies, 
fall to the ground? 

Mrs. B. On account of its spring or elasticity. The air 
is an elastic fluid; a species of bodies, the peculiar property 
of whjch is to resume, after compression, their original di¬ 
mensions; and you must consider the air of which the at¬ 
mosphere is composed as existing in a state of compression, 
for its particles being drawn towards the earth by gravity, 
are brought closer together than they would otherwise be, 
but the spring or elasticity of the air by which it endeavours 
to resist compression, gives it a constant tendency to expand 
itself, so as to resume the dimensions it would naturally 
have, if not under the influence of gravity. The air may 
therefore be said constantly to struggle with the power of 
gravity without being able to overcome it. Gravity thus 
confines the air to the regions of our globe, whilst its elasti¬ 
city prevents it from falling like other bodies to the ground. 


32 


OP THE ATTRACTION OP GRAVITY. 


Emily . The air then is, I suppose, thicker, or I should 
rather say more dense, near the surface of the earth, than 
in the higher regions of the atmosphere; for that part of the 
air which is nearer the surface of the earth must be most 
strongly attracted. 

Mrs. B. The diminution of the force of gravity, at so 
small a distance as that to which the atmosphere extends 
(compared with the size of the earth) is so inconsiderable 
as to be scarcely sensible; but the pressure of the upper 
parts of the atmosphere on those beneath, renders the air 
near the surface of the earth much more dense than the 
upper regions. The pressure of the atmosphere has been 
compared to that of a pile of fleeces of wool, in wdiich the 
lower fleeces are pressed together by the weight of those 
above; these lie light and loose, in proportion as thdy ap¬ 
proach the uppermost fleece, which receives no External 
pressure, and is confined merely by the force of its own 
gravity. 

Caroline. It has just occurred to me that there are some 
bodies which do not gravitate towards the earth. Smoke 
and steam, for instance, rise instead of falling. 

Mrs B. It is still gravity which produces their ascent; 
at least, were that power destroyed, these bodies would not 
rise. 

Caroline. I shall be out of conceit with gravity, if it is 
so inconsistent in its operations. 

Mrs. B. There is no difficulty in reconciling this appa¬ 
rent inconsistency of effect. The air near the earth is hea¬ 
vier than smoke, steam, or other vapours; it consequently 
not only supports these light bodies, but forces them to rise, 
till they reach a part of the atmosphere, the weight of which 
is not greater that their own, and then they remain station¬ 
ary. Look at this bason of water; why does the piece of 
paper which I throw into it float on the surface? 

Emily. Because, being lighter than the water, it is sup¬ 
ported by it, 

Mrs. B. And now that I pour more water into the ba¬ 
son, why does the paper rise? 

Emily. The water being heavier than the paper, gets be¬ 
neath it, and obliges it to rise. 


ON THE ATTRACTION OF GRAVITY. 33 

Mrs. B. In a similar manner are smoke and vapour 
forced upwards by the air; but these bodies do not, like the 
paper ascend to the surface of the fluid, because, as we ob¬ 
served before, the air being thinner and lighter as it is more 
distant from the earth, vapours rise only till they attain a 
region of air of their own density. Sr»oke, indeed ascends 
but a very little way; it consists of minute particles of fuel 
carried up by a current of heated air from the fire below: 
heat, you recollect, expand? all bodies; it consequently rare¬ 
fies air, and renders it I%titer than the colder air of the at¬ 
mosphere; the heated air from tIle ^ re carries up with it 
vapour and sm^ particles of the combustible materials 
which are burning in the fire. When this current of hot 
air is cooUd by mixing with the atmosphere, the minute 
particM of coal or other combustible fail; it is this which 
produces the small black flakes which render the air, and 
every thing in contact with it, in London, so dirty. 

Caroline. You must, however, allow me to make one 
more objection to the universal gravity of bodies; which is 
the ascent of air balloons, the materials of which are un¬ 
doubtedly heavier than air: how, therefore, can they be 
supported by it? 

Mrs. B. I admit that the materials of which balloons 
are made are heavier than the air; but the air with which 
they are filled is an elastic fluid, of a different nature from the 
atmospheric air, and considerably lighter; so that on the 
whofr the balloon is lighter that the air which it displaces, 
and consequently will rise, on the same principle as smoke 
and vapour. Now, Emily, let me hear if you can explain 
how the gravity of bodies is modified by the effect of the air? 

Emily. The air forces bodies which are lighter than it¬ 
self to ascend; those that are of an equal weight will remain 
stationary in it; and those that are heavier will descend 
through it: but the air will have some effect on these last; for 
if they are not much heavier, they will with difficulty over¬ 
come the resistance they meet with in passing through it, 
they will be borrife up by it, and their fall will be more or 
less retarded. 

Mrs. B . Very well. Observe how slowly this light fea» 

4 


U ON THE ATTRACTION OF GRAVITY. 

ther falls to the ground, while a heavier body, like this mar¬ 
ble, overcomes the resistance which the air makes to its 
descent much more easily, and its fall is proportionally more 
rapid. I now throw a pebble into this tub of water; it does 
not reach the bottom near so soon as if there were no wa¬ 
ter in the tub, because it meets with resistance from the 
water. Suppose that we could empty the tub, not only of 
water, but of air also, the pebble would then fall quicker 
still, as it would in that case mt*t with no resistance at all 
to counteract its gravity. 

Thus you see that it is not the diffei^t degrees of gravi¬ 
ty, but the resistance of the air, which prevents bodies of 
different weight from falling with equal velockles; if the air 
did not bear up the feather, it would reach the ground as 
soon as the marble. 

Caroline. I make no doubt that it is so; and yet i do 
not feel quite satisfied. I wish there was any place voiCi 
of air, in which the experiment could be made. 

Mrs. B. If that proof will satisfy your doubts, I can 
give it you. Here is a machine called an air pump , (fig. 
2, pi. 1,) by means of which the air may be expelled from 
any close vessel which is placed over this opening, through 
which the air is pumped out. Glasses of various shapes, 
usually called receivers, are employed for this purpose* We 
shall now exhaust the air from this tall receiver which is 
placed over the opening, and we shall find that bodies of 
whatever weight or size within it, will fall from the top to 
the bottom in the same space of time. 

Caroline. Oh, I shall be delighted with this experiment; 
what a curious machine! how can you put the two bodies 
of different weight within the glass, without admitting the 
air? 

Mrs. B. A guinea and a feather are already placed there 
for the purpose of the experiment: here is, you see, a con¬ 
trivance to fasten them in the upper part of the glassf as 
soon as the air is pumped out, I shall turn this little screw, 
by which means the brass plates which support them will 
be inclined, and the two bodies will fall,—Now I believe 
I have pretty well exhausted the air. 


ON THE ATTRACTION OF GRAVITY. 


35 


Caroline. Pray let me turn the screw.—I declare, they 
both reached the bottom at the same instant! Did you see, 
Emily, the feather appeared as heavy as the guinea? 

Emily, Exactly; and fell just as quickly. How won¬ 
derful this is! what a number of entertaining experiments 
might be made with this machine! 

Mrs, B, No doubt there are a great variety; but we shall 
reserve them to elucidate the subjects to which they relate: 
if I had not explained to you why the guinea and the fea¬ 
ther fell with equal velocity, you would not have been so 
well pleased with the experiment. 

Emily. I should have been as much surprised, but not so 
much interested; besides, experiments help to imprint on 
the memory the facts they are intended to illustrate; it will 
be better therefore for us to restrain our curiosity, and wait 
for other experiments in their proper places. 

Caroline. Pray by what means is the air exhausted in 
this receiver? 

Mrs, B. You must learn something of mechanics in or¬ 
der to understand the construction of a pump. At our next 
meeting, therefore, I shall endeavour to make you acquaint¬ 
ed with the laws of motion, as an introduction to that sub¬ 
ject 


CONVERSATION Ill 


ON THE LAWS OF MOTION. 


OF .MOTION.—OF TIIE INERTIA OF BODIES.-OF FORCE TO PRODUCE MOTION. 

-DIRECTION OF MOTION.-VELOCITY, ABSOLUTE AND RELATIVE.- 

UNIFORM MOTION.-RETARDED MOTION.-ACCELERATED MOTION.- 

VELOCITY OF FALLING BODIES.-MOMENTUM.-ACTION AND REACTION 

EQ.UAL.-ELASTICITY OF BODIES.-rOROSITY OF BODIES.-REFLECTED 

MOTION.-ANGLES OF INCIDENCE AND REFLECTION. 


MRS. B. 

The science of mechanics is founded on the laws of mo¬ 
tion; it will therefore be necessary to make you acquainted 
with these laws before we examine the mechanical powers. 
Tell me, Caroline, what do you understand by the word 
motion? 

Caroline. I think I understand it perfectly, though I am 
at a loss to describe it. Motion is the act of moving about, 
of going from one place to another, it is the contrary of re¬ 
maining at rest. 

Mrs. B. Very well. Motion then consists in a change 
of place; a body is in motion whenever it is changing its 
situation with regard to a fixed point. 

Now since we have observed that one of the general pro¬ 
perties of bodies is Inertia, that is, an entire passiveness, 
either with regard to motion or rest, it follows that a body 
can not move without being put into motion; the power 
TVhich puts a body into motion is called force; thus the stroke 
of the hammer is the force which drives the nail; the pull¬ 
ing of the horse that which draws the'carriage, &c. Force 
then is the cause which produces motion. 


ON THE LAWS OF MOTION. 37 

Emily. And may we not say that gravity is the force 
which occasions the fall of bodies? 

Mrs. B. Undoubtedly. I have given you the most fa¬ 
miliar illustrations in order to render the explanation clear; 
but since you seek for more scientific examples, you may 
say that cohesion is the force which binds the particles of 
bodies together, and heat that which drives them asunder. 

The motion of a body acted upon by a single force is al¬ 
ways in a straight line, in the direction in which it received 
the impulse. 

Caroline . That is very natural; for as the body is inert, 
and can move only because it is impelled, it will move on¬ 
ly in the direction in which it is impelled. The degree of 
quickness with which it moves, must, I suppose, also de¬ 
pend upon the degree of force with which it is impelled. 

Mrs. B. Yes; the rate at which a body moves, or the 
shortness of the time which it takes to move from one place 
to another, is called its velocity; and it is one of the laws 
of motion, that the velocity of the moving body is propor¬ 
tional to the force by which it is put in motion. We must 
distinguish between absolute and relative velocity. 

The velocity of a body is called absolute , if we consider 
the motion of the body in space, without any reference to 
that of other bodies. When, for instance, a horse goes fifty 
miles in ten hours, his velocity is five miles an hour. 

The velocity of a body is termed relative , when compared 
with that of another body which is itself in motion. For 
instance, if one man walks at the rate of a mile an hour, 
and another at the rate of two miles an hour, the relative 
velocity of the latter is double that of the former; but the 
absolute velocity of the one is one mile, and that of the other 
two miles an hour. 

Emily. Let me see if I understand it—The relative ve¬ 
locity of a body is the degree of rapidity of its motion com¬ 
pared with that of another body; thus if one ship sail three 
times as far as another ship in the same space of time, the 
velocity of the former is equal to three times that of the latter. 

Mrs. B. The general rule may be expressed thus: the 
velocity of a body is measured by the space over which it 
4 * 


38 


ON THE LAWS OF MOTION. 


moves, divided by the time which it employs in that mo¬ 
tion: thus if you travel one hundred miles in twenty hours, 
what is your velocity in each hour? 

Emily. I must divide the space, which is one hundred 
miles, by the time, which is twenty hours, and the answer 
will be five miles an hour. Then, Mrs. B., may we not re¬ 
verse this rule and say that the time is equal to the space 
divided by the velocity; since the space, one hundred miles, 
divided by the velocity, five miles, gives twenty hours for 
the time? 

Mrs. B. Certainly; and we may say also that space is 
equal to the velocity multiplied by the time. Can you tell 
me, Caroline, how many miles you will have travelled, if 
your velocity is three miles an hour, and you travel six hours? 

Caroline. Eighteen miles; for the product of 3 multi¬ 
plied by 6, is 18. 

Mrs. B. I suppose that you understand what is meant by 
the terms uniform , accelerated and retarded motion. 

Emily. I conceive uniform motion to be that of a body 
whose motion is regular, and at an equal rate throughout; 
for instance, a horse that goes an equal number of miles every 
hour. But the hand of a watch is a much better example, as 
its motion is so regular as to indicate the time. 

Mrs. B. You have a right idea of uniform motion; but 
it would be more correctly expressed by saying, that the 
motion of a body is uniform when it passes over equal spaces 
in equal times. Uniform motion is produced by a force hav¬ 
ing acted on a body once and having ceased to act; as, for 
instance, the stroke of a bat on a cricket ball. 

Caroline. But the motion of a cricket ball is not uni¬ 
form; its velocity gradually diminishes till it falls to the 
ground. 

Mrs. B. Recollect that the cricket ball is inert, and 
has no more power to stop than to put itself in motion; if it 
falls, therefore, it must be stopped by some force superior 
to that by which it was projected, and which destroys its 
motion. 

Caroline . And it is no doubt the force of gravity which 
counteracts and destroys that of projection; but if there were 
no such power as gravity, would the cricket ball never stop? 


ON THE LAWS OP MOTION. 


39 


Mrs* B. If neither gravity nor any other force, such as 
the resistance of the air, opposed its motion, the cricket 
ball, or even a stone thrown by the hand, would proceed on¬ 
wards in a right line, and with a uniform velocity forever. 

Caroline . You astonish me! I thought that it was im¬ 
possible to produce perpetual motion? 

Mrs. B. Perpetual motion can not be produced by art, 
because gravity ultimately destroys all motion that human 
powers can produce. 

Emily. But independently of the force of gravity, I can 
not conceive that the little motion I am capable of giving to 
a stone would put it in motion forever. 

Mrs. B. The quantity of motion you communicate to 
the stone would not influence its duration; if you threw it 
with little force it would move slowly, for its velocity you 
must remember, will be proportional to the force with which 
it is projected; but if there is nothing to obstruct its passage, 
it will continue to move with the same velocity, and in the 
same direction as when you first projected it. 

Caroline. This appears to me quite incomprehensible; 
we do not meet with a single instance of it in nature. 

Mrs. B. I beg your pardon. When you come to study 
the motion of the celestial bodies, you will find that nature 
abounds with examples of perpetual motion; and that it con¬ 
duces as much to the harmony of the system of the universe, 
as the prevalence of it would to the destruction of all com¬ 
fort on our globe. The wisdom of Providence has there¬ 
fore ordained insurmountable obstacles to perpetual motion 
here below, and though these obstacles often compel us to 
contend with great difficulties, yet there results from it that 
order, regularity and repose, so essential to the preservation 
of all the various beings of which this world is composed. 

Now can you tell me what is retarded motion ? 

Caroline. Retarded motion is that of a body which 
moves every moment slower and slower: thus when I am 
tired with walking fast, I slacken my pace; or when a stone 
is thrown upwards, its velocity is gradually diminished by 
the power of gravity. 


40 


ON THE LAWS OF MOTION. 


Mrs. B. Retarded motion is produced by some force 
acting upon the body in a direction opposite to that which 
first put it in motion: you who are an animated being, en¬ 
dowed with power and will, may slacken your pace, or stop 
to rest when you are tired; but inert matter is incapable of 
any feeling of fatigue, can never slacken its pace, and never 
stop unless retarded or arrested in its course by some op¬ 
posing force; and as it is the laws of inert bodies which 
mechanics treats of, I prefer your illustration of the stone 
retarded in its ascent. Now Emily, it is your turn; what is 
accelerated motion ? 

Emily. Accelerated motion, I suppose, takes place when 
the velocity of a body is increased; if you had not objected 
to our giving such active bodies as ourselves ^s examples, I 
should say that my motion is accelerated if"I change my 
pace from walking to running. I can not think of any in¬ 
stance of accelerated motion in inanimate bodies; all mo¬ 
tion of inert matter seems to be retarded by gravity. 

Mrs . B. Not in all cases; for the power of gravitation 
sometimes produces accelerated motion; for instance, a stone 
falling from a height moves with a regularly accelerated 
motion. 

Emily. True; because the nearer it approaches the earth, 
the more it is attracted by it. 

Mrs. B. You have mistaken the cause of its accelera¬ 
tion of motion; for though it is true that the force of gravity 
increases as a body approaches the earth, the difference is 
so trifling at any small distance from its surface as not to be 
perceptible. 

Accelerated motion is produced when the force which put 
a body in motion continues to act upon it during its motion, 
so that its motion is continually increased. When a stone 
falls from a height, the impulse which it receives from gra¬ 
vity during the first instant of its fall, would be sufficient to 
bring it to the ground with a uniform velocity: for, as we 
have observed, a body having been once acted upon by a 
force, will continue to move with a uniform velocity; but the 
stone is not acted upon by gravity merely at the first instant 
of its fall; this power continues to impel it during the whole 


ON THE LAWS OF MOTION. 41 

of its descent, and it is this continued impulse which ac¬ 
celerates its motion. 

Emily. I do not quite understand that. 

Mrs. B. Let us suppose that the instant after you have 
let fall a stone from a high tower, the force of gravity were 
annihilated, the body would nevertheless continue to move 
downwards, for it would have received a first impulse from 
gravity, and a body once put in motion will not stop unless 
it meets with some obstacle to impede its course; in this 
case its velocity would be uniform, for though there would 
be no obstacle to obstruct its descent, there would be no 
force to accelerate it. 

Emily. That is very clear. 

Mrs . B. Then you have only to add the power of gravity 
constantly acting on the stone during its descent, and it will 
not be difficult to understand that its motion will become 
accelerated, since the gravity which acts on the stone dur¬ 
ing the first instant of its descent, will continue in force every 
instant till it reaches the ground. Let us suppose that the 
impulse given by gravity to the stone during the first instant 
of its descent be equal to one, the next instant we shall find 
that an additional impulse gives the stone an additional ve¬ 
locity equal to one, so that the accumulated velocity is now 
equal to two; the following instant another impulse increases 
the velocity to three, and so on till the stone reaches the 
ground. 

Caroline. Now I understand it; the effects of preceding 
impulses must be added to the subsequent velocities. 

Mrs. B. Yes; it has been ascertained, both by experi¬ 
ment and calculations, which it would be too difficult for us 
to enter into, that heavy bodies descending from a height by 
the force of gravity, fall sixteen feet the first second of time, 
three times that distance in the next, five times in the third 
second, seven times in the fourth, and so on, regularly in¬ 
creasing their velocities according to the number of seconds 
during which the body has been falling. 

Emily. If you throw a stone perpendicularly upwards, 
is it not the same length of time ascending that it is de¬ 
scending? 


42 


ON THE LAWS OF MOTION, 


Mrs. B. Exactly; in ascending, the velocity is dimi¬ 
nished by the force of gravity; in descending, it is accele¬ 
rated by it. 

Caroline. I should then have imagined that it would 
have fallen quicker than it rose? 

Mrs. B. You must recollect that the force with which 
it is projected must be taken into the account; and that this 
force is overcome and destroyed by gravity before the body 
falls. 

Caroline. But the force of projection given to a stone in 
throwing it upwards, can not always be equal to the force of 
gravity in bringing it down again, for the force of gravity is 
always the same, whilst the degree of impulse given to the 
stone is optional; I may throw it up gently, or with violence. 

Mrs. B. If you throw it gently, it will not rise high; 
perhaps only sixteen feet, in which case it will fall in one 
second of time. Now it is proved by experiment, that an 
impulse requisite to project a body sixteen feet upwards, 
will make it ascend that height in one second; here then 
the times of the ascent and descent are equal. But sup¬ 
posing it be required to throw a stone twice that height, the 
force must be proportionally greater. 

You see then, that the impulse of projection in throwing 
a body upwards, is always equal to the action of the force 
of gravity during its descent; and that it is the greater or 
less distance to which the body rises, that makes these two 
forces balance each other. 

I must now explain to you what is meant by the momen¬ 
tum of bodies. It is the force, or power, with which a body 
in motion, strikes against another body. The momentum of 
a body is composed of its quantity of matter , multiplied by 
its quantity of motion; in other words, its weight and its ve¬ 
locity. 

Caroline. The quicker a body moves, the greater, no 
doubt, must be the force with which it would strike against 
another body. 

Emily . Therefore a small body may have a greater mo¬ 
mentum than a large one, provided its velocity be sufficiently 


ON THE LAWS OF MOTION. 


43 


greater; for instance, the momentum of an arrow shot from 
a bow, must be greater than a stone thrown by the hand. 

Caroline . We know also by experience, that the heavier 
a body is, the greater is its force; it is not therefore difficult 
to understand, that the whole power or momentum of a body 
must be composed of these two properties: but I do not un¬ 
derstand why they shouid be multiplied , the one by the other; 
I should have supposed that the quantity of matter should 
have been added to the quantity of motion? 

Mrs. B. It is found by experiment, that if the weight of 
a body is represented by the number 3, and its velocity also 
by 3, its momentum will be represented by 9; not 6, as 
would be the case, were these figures added, instead of be¬ 
ing multiplied together. I recommend it to you to be care¬ 
ful to remember the definition of the momentum of bodies, 
as it is one of the most important points in mechanics; you 
will find, that it is from opposing motion to matter, that ma¬ 
chines derive their powers.* 

The reaction of bodies, is the next law of motion which 
I must explain to you. When a body in motion strikes 
against another body, it meets with resistance from it; the 
resistance of the body at rest will be equal to the blow struck 
by the body in motion; or to express myself in philosophical 
language, action and reaction will be equal, and in oppo¬ 
site directions. 

Caroline. Do you mean to say, that the action of the 
body which strikes, is returned with equal force by the body 
which receives the blow? 

J\frs. B. Exactly. 

Caroline. But if a man strikes another on the face with 
his fist, he surely does not receive as much pain by the re¬ 
action, as he inflicts by the blow? 

* In comparing together the momenta of different bodies, we must 
be attentive to measure their weights and velocities, by the same de¬ 
nomination ot weights and of spaces, otherwise the results would not 
agree. Thus if we estimate the weight of one body in ounces, we 
must estimate the weight of the rest also in ounces, and not in pounds; 
and in computing the velocities, in like manner we should adhere to 
the same standard of measure, both of space and of time; as for in¬ 
stance, the number of feet in one second, or of miles in one hour. 


44 


ON THE LAWS OF MOTION. 


Mrs. B. No; but this is simply owing to the knuckles 
having much less feeling than the face. 

Here are two ivory balls suspended by threads, (plate I. 
fig. 3.) draw one of them, A, a little on one side,—now let 
it go;—it strikes, you see, against the other ball B, and 
drives it off, to a distance equal to that through which the 
first ball fell; but the motion of A is stopped, because when 
it struck B, it received in return a blow equal to that it gave, 
and its motion was consequently destroyed. 

Emily. I should have supposed, that the motion of the 
ball A was destroyed, because it had communicated all its 
motion to B. 

Mrs. B. It is perfectly true, that when one body strikes 
against another, the quantity of motion communicated to the 
second body, is lost by the first; but this loss proceeds from 
the action of the body which is struck. 

Here are six ivory balls hanging in a row, (fig. 4.) draw 
the first out of the perpendicular, and let it fall against the 
second. None of the balls appear to move, you see, except 
the last, which flies off as far as the first ball fell; can you 
explain this? 

Caroline. I believe so. When the first ball struck the 
second, it received a blow in return, which destroyed its 
motion; the second ball, though it did not appear to move, 
must have struck against the third; the reaction of which 
set it at rest; the action of the third ball must have been de¬ 
stroyed by the reaction of the fourth, and so on till motion 
was communicated to the last ball, which, not being re¬ 
acted upon, flies off. 

Mrs. B. Very well explained. Observe, that it is only 
when bodies are elastic, as these ivory balls are, that the 
stroke returned is equal to the stroke given. I will show 
you the difference with these two balls of clay, (fig. 5.) 
which are not elastic; when you raise one of these, D, out 
of the perpendicular, and let it fall against the other, E, the 
reaction of the latter, on account of its not being elastic, is 
not sufficient to destroy the motion of the former; only part 
of the motion of D will be communicated to E, and the two 


ON THE LAWS OF MOTION. 4o 

balls will move on together to d and e, which is not so great 
a distance as that through which D tell. 

Observe how useful reaction is in nature. Birds in fly¬ 
ing strike the air with their wings, and it is the reaction of 
the air which enables them to rise, or advance forwards; 
reaction being always in a contrary direction to action. 

Caroline. I thought that birds might be lighter than the 
air, when their wings were expanded, and by that means 
enabled to fly. 

Mrs. B. When their wings arc spread, they are better 
supported by the air, as they cover a greater extent of sur¬ 
face; but they are still much too heavy to remain in that 
situation, without continually flapping their wings, as you 
may have noticed when birds hover over their nests: the 
force with which their wings strike against the air must 
equal the weight of their bodies, in order that the reaction 
of the air may be able to support that weight; the bird will 
then remain stationary. If the stroke of the wings is great¬ 
er than is required merely to support the bird, the reaction 
of the air will make it rise; if it be less, it will gently de¬ 
scend; and you may have observed the lark, sometimes 
remaining with its wings extended, but motionless: in this 
state it drops rapidly into its nest. 

Caroline. What a beautiful effect this is of the law of 
reaction! But if flying is merely a mechanical operation, 
Mrs. B., why should we not construct wings, adapted to the 
size of our bodies, fasten them to our shoulders, move them 
with our arms, and soar into the air? 

Mrs. B. Such an experiment has been repeatedly at¬ 
tempted, but never with success; and it is now considered 
as totally impracticable. The muscular power of birds is 
greater in proportion to their weight than that of man; were 
we therefore furnished with wings sufficiently large to en¬ 
able us to fly, we should not have strength to put them in 
motion. 

In swimming, a similar action is produced on the water, 
as that on the air in flying; and also in rowing; you strike 
the water with the oars, in a direction opposite to that in 


4t> 


ON THE LAWS OF MOTION. 


which the boat is required to move, and it is the reaction of 
the water on the oars which drives the boat along. 

Emily . You said, that it was in elastic bodies only, that 
reaction was equal to action; pray what bodies are elastic 
besides the air? 

Mrs. B. In speaking of the air, I think we defined elas¬ 
ticity to be a property, by means of which bodies that are 
compressed returned to their former state. If I bend this 
cane, as soon as I leave it at liberty it recovers its former 
position; if I press my finger upon your arm, as soon as I 
remove it, the flesh, by virtue of its elasticity, rises and de¬ 
stroys the impression I made. Of all bodies, the air is the 
most eminent for this property, and it has thence obtained 
the name of elastic fluid. Hard bodies axe in the next de¬ 
gree elastic; if two ivory, or metallic balls are struck toge¬ 
ther, the parts at which they touch will be flattened; but 
their elasticity will make them instantaneously resume their 
former shape. 

Caroline. But when two ivory balls strike against each 
other, as they constantly do on a billiard table, no mark or 
impression is made by the stroke. 

Mrs. B. I beg your pardon; but you can not perceive 
any mark, because their elasticity instantly destroys all 
trace of it. 

Soft bodies, which easily retain impressions, such as clay, 
wax, tallow, butter, &c. have very little elasticity; but of 
all descriptions of bodies liquids are the least elastic. 

Emily. If sealing-wax were elastic, instead of retaining 
the impression of a seal, it would resume a smooth surface 
as soon as the weight of the seal was removed. But pray 
what is it that produces the elasticity of bodies? 

Mrs. B. There is great diversity of opinion upon that 
point, and I can not pretend to decide which approaches 
nearest to the truth. Elasticity implies susceptibility of 
compression, and the susceptibility of compression depends 
upon the porosity of bodies, for were there no pores or 
spaces between the particles of matter of which a body is 
composed, it could not be compressed. 

Caroline. That is to say, that if the particles of bodies 


ON THE LAWS OF MOTION. 47 

were as dose together as possible, they could not be 
squeezed closer. 

Emily. Bodies then, whose particles are most distant 
from each other, must be most susceptible of compression, 
and consequently most elastic; and this you say is the case 
with air, which is perhaps the least dense of all bodies? 

Mrs. B. You will not in general find this rule hold good, 
for liquids have scarcely any elasticity, whilst hard bodies 
are eminent for this property, though the latter are certain¬ 
ly of much greater density than the former; elasticity im¬ 
plies, therefore, not only a susceptibility of compression, 
but depends upon the power of resuming its former state 
after compression. 

Caroline. But surely there can be no pores in ivory 
and metals, Mrs. B ; how then can they be susceptible of 
compression? 

Mrs. B. The pores of such bodies are invisible to the 
naked eye, but you must not thence conclude that they 
have none; it is, on the contrary, well ascertained that gold, 
one of the most dense of all bodies, is extremely porous, 
and that these pores are sufficiently large to admit water 
when strongly compressed to pass through them. This was 
shown by a celebrated experiment made many years ago at 
Florence. 

Emily. If water can pass through gold, there must cer¬ 
tainly be pores or interstices which afford it a passage; and 
if gold is so porous, what must other bodies be which are 
so much less dense than gold! 

Mrs. B. The chief difference in this respect is, I be¬ 
lieve, that the pores in some bodies are larger than in others; 
in cork-, sponge and bread, they form considerable cavities; 
iri wood and stone, when not polished, they are generally 
perceptible to the naked eye; whilst in ivory, metals, and 
all varnished and polished bodies, they can not be discern¬ 
ed. To give you an idea of the extreme porosity of bodies, 
sir Isaac Newton conjectured that if the earth were so com¬ 
pressed as to be absolutely without pores, its dimensions 
might possibly not be more than a cubic inch. 

Caroline. What an idea! Were we not indebted to sir 


48 


ON THE LAWS OF MOTION. 


Isaac Newton for the theory of attraction, I should be tempt¬ 
ed to laugh at him for such a supposition. What insignifi¬ 
cant little creatures we should be! 

Mrs. B. If our consequence arose from the size of our 
bodies, we should indeed be but pigmies, but remember 
that the mind of Newton was not circumscribed by the di¬ 
mensions of its envelope. 

Emily. It is, however, fortunate that heat keeps the pores 
of matter open and distended, and prevents the attraction 
of cohesion from squeezing us into a nut-shell. 

Mrs. B. Let us now return to v the subject of reaction, 
on which we have some further observations to make. It 
is reaction, being contrary to action, which produces reflect¬ 
ed motion. If you throw a ball against th'fc wall, it rebounds; 
this return of the ball is owing to the reaction of the wall 
against which it struck, and is called reflected motion . 

Emily. And I now understand why balls filled with air 
rebound better than those stuffed with bran or wool, air 
being most susceptible of compression and most elastic, the 
reaction is more complete. 

Caroline. I have observed that when I throw a ball 
straight against the wall, it returns straight to my hand; but 
if 1 throw it obliquely upwards, it rebounds still higher, 
and I catch it when it falls. 

Mrs . B. You should not say straight, but perpendicu¬ 
larly against the wall; for straight is a general term for 
lines in all directions which are neither curved nor bent, 
and is therefore equally applicable to oblique or perpendi¬ 
cular lines. 

Caroline. I thought that perpendicularly meant either 
directly upwards or downwards? 

Mrs. B . In those directions lines are perpendicular to 
the earth. A perpendicular line has always a reference to 
something towards which it is perpendicular; that is to say, 
that it inclines neither to the one side or the other, but 
makes an equal angle on every side. Do you understand 
what an angle is? 

Caroline. Yes, I believe so; it is two lines meeting in a 
point. 






















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IYati: it. 














































ON THE LAWS OP MOTION. 


49 


Mrs, B. Well then, let the line A B (plate II. fig. 1.) 
represent the floor of the room, and the line C D that in 
which you throw a ball against it; the line C D, you will 
observe, forms two angles with the line A B, and those two 
angles are equal. 

Emily. How can the angles be equal, while the line* 
which compose them are of unequal length? 

Mrs. B. An angle is not measured by the length of the 
lines, but by their opening. 

Emily. Yet the longer the lines are, the greater is the 
opening between them. 

Mrs . B. Take a pair of compasses and draw a circle 
over these angles, making the angular point the centre. 

Emily. To what extent must I open the compasses? 

Mrs. B. You may draw the circle what size you please, 
provided that it cuts the lines of the angles we are to mea¬ 
sure. All circles, of whatever dimensions, are supposed to 
be divided into 360 equal parts, called degrees; the opening 
of an angle, being therefore a portion of a circle, must con¬ 
tain a certain number of degrees: the larger the angle the 
greater number of degrees,and the two angles are said to 
be equal when they contain an equal number of degrees. 

Emily. Now I understand it. As the dimensions of an 
angle depend upon the number of degrees contained between 
its lines, it is the opening, and not the length of its lines, 
which determines the size of the angle. 

Mrs. B. Very well: now that you have a clear idea of the 
dimensions of angles, can you tell me how many degrees 
are contained in the two angles formed by one line falling 
perpendicular on another, as in the figure I have just drawn? 

Emily. You must allow me to put one foot of the com¬ 
passes at the point of the angles, and draw a circle round 
them, and then I think I shall be able to answer your ques* 
tion: the two angles are together just equal to half a circle, 
they contain therefore 90 degrees each; 90 degrees being a 
quarter of 360. 

Mrs. B. An angle of 90 degrees is called a right an¬ 
gle, and when one line is perpendicular to another, it forms, 
you see, (fig. 1.) a right angle on either side. Angles con- 
5 - 


50 


ON THE LAWS OF MOTION. 


taining more than 90 degrees are called obtuse angles, (fig. 
2.) and those containing less than 90 degrees are called 
acute angles, (fig. 3.) 

Caroline. The angles of this square table are right an¬ 
gles, but those of the octagon table are obtuse angles; and 
the angles of sharp-pointed instruments are acute angles. 

Mrs. B. Very well. To return now to your observa¬ 
tion, that if a ball is thrown obliquely against the wall, it 
will not rebound in the same direction; tell me, have you 
ever played at billiards? 

Caroline. Yes, frequently; and I have observed that 
when I push the ball porpendicularly against the cushion, it 
returns in the same direction; but when I send it obliquely 
to the cushion, it rebounds obliquely, 'but on an opposite 
side; the ball in this latter case describes an angle, the point 
of which is at the cushion. I have observed too, that the 
more obliquely the ball is struck against the cushion, the 
more obliquely it rebounds on the opposite side, so that a 
billiard player can calculate with great accuracy in what 
direction it will return. 

Mrs . B. Very well. This figure (fig. 4, plate II.) re¬ 
presents a billiard table; now if you draw a line A B from 
the point where the ball A strikes perpendicular to the cush¬ 
ion, you will find that it will divide the angle which the ball 
describes into two parts, or two angles; the one will show 
the obliquity of the direction of the ball in its passage to¬ 
wards the cushion, the other its obliquity in its passage back 
from the cushion. The first is called the angle of incidence , 
the other the angle the angle of reflection , and these angles 
are always equal. 

Caroline. This then is the reason why, when I throw 
a ball obliquely against the wall, it rebounds in an opposite 
oblique direction, forming equal angles of incidence and of 
reflection. 

Mrs. B. Certainly; and you will find that the more 
obliquely you throw the ball, the more obliquely it will re¬ 
bound. 

We must now conclude; but I shall have some further ob¬ 
servations to make upon the laws of motion, at our next 
meeting. 


CONVERSATION IV. 


ON COMPOUND MOTION. 


COMPOUND MOTION, THE RESULT OF TWO OPPOSITE FORCES.-OF CIRCULAR 

MOTION, THE RESULT OF TWO FORCES, ONE OF WHICH CONFINES THE 

BODY TO A FIXED POINT.-CENTRE OF MOTION, THE POINT AT REST 

WHILE THE OTHER PARTS OF THE BODY MOVE ROUND IT.-CENTRE OF 

MAGNITUDE, THE MIDDLE OF A BODY.-CENTRIPETAL FORCE, THAT 

WHICH CONFINES A BODY TO A FIXED CENTRAL POINT.-CENTRIFUGAL 

FORCE, THAT WHICH IMPELS A BODY TO FLY EROM THE CENTRE.—FALL 

OF BODIES IN A PARABOLA.-CENTRE OF GRAVITY, THE CENTRE OF 

WEIGHT, OR POINT ABOUT WHICH THE PARTS BALANCE EACH OTHER. 


MRS. B. 

* \/ f 

I must now explain to you the nature of compound mo¬ 
tion. Let us suppose a body to be struck by two equal 
forces in opposite directions, how will it move? 

Emily. If the directions of the forces are in exact oppo¬ 
sition to each other, I suppose the body would not move at 
aH. . 

Mrs. B. You are perfectly right; but if the forces, in¬ 
stead of acting on the body in opposition, strike it in two 
directions, inclined to each other, at an angle of ninety de¬ 
grees, if the ball A (fig. 5, plate II.) be struck by equal 
forces at X and at Y, will it not move? 

Emily. The force X would send it towards B, and the 
force Y towards C; and since these forces are equal, I do 
not know how the body can obey one impulse rather than 
the other, and yet I think the ball would move, because as 
the two forces do not act in direct opposition, they can not 
entirely destroy the effect of each other. 


52 


ON COMPOUND MOTION. 


Mrs. B. Very true; the ball will therefore follow the 
direction of neither of the forces, but will move in a line 
between them, and will reach D in the same space of time, 
that the force X would have sent it to B, and the force Y 
would have sent it to C. Now if you draw two lines from 
D, to join B and C, you will form a square, and the oblique 
line which the body describes is called the diagonal of the 
square. 

Caroline. That is very clear, but supposing the two 
forces to be unequal, that the force X, for instance, be twice 
as great as the force Y? 

Mrs. B. Then the force X would drive the ball twice 
as far as the force Y, consequently you must draw the line 
A B (fig. 6.,) twice as long as the line A C, the body will 
in this case move to D; and if you draw lines from that point 
to B and C, you will find that the ball has moved in the 
diagonal of a rectangle. 

Emily. Allow me to put another case? Suppose the two 
forces are unequal, but do not act on the ball in the direc¬ 
tion of a right angle, but in that of an acute angle, what will 
result? 

Mrs. B. Prolong the lines in the directions of the two 
forces, and you will soon discover which way the ball will 
be impelled; it will move from A to D, in the diagonal of a 
parallelogram, (fig. 7.) Forces acting in the direction of 
lines forming an obtuse angle, will also produce motion in 
the diagonal of a parallelogram. For instance, if the body 
set out from B, instead of A, and was impelled by the forces 
X and Y, it would move in the dotted diagonal B C. 

We may now proceed to circular motion: this is the re¬ 
sult of two forces on a body, by one of which it is projected 
forward in a right line, whilst by the other it is confined to 
a fixed point. For instance, when I whirl this ball, which is 
fastened to my hand with a string, the ball moves in a cir¬ 
cular direction; because it is acted on by two forces, that 
which I give it which represents the force of projection, and 
that of the string which confines it to my hand. If during its 
motion you were suddenly to cut the string, the ball would 
fly off in a straight line; being released from confinement to 


ON COMPOUND MOTION. 


53 


the fixed point, it would be acted on but by one force, and 
motion produced by one force, you know, is always in a 
right line. 

Caroline. This is a little more difficult to comprehend 
than compound motion in straight lines. 

Mrs. B. You have seen a mop trundled, and have ob¬ 
served, that the threads which compose the head of the mop 
fly from the centre; but being confined to it atone end, they 
can not part from it; whilst the water they contain, being 
unconfined, is thrown off in straight lines. 

Emily. In the same way, the flyers of a windmill, when 
put in motion by the wind, would be driven straight forwards 
in a right line, were they not confined to a fixed point, round 
which they are compelled to move. 

Mrs. B. Very well. And observe, that the point to which 
the motion of a small body, such as the ball with the string, 
which may be considered as revolving in one plane, is con¬ 
fined, becomes the centre of its motion. But when the bodies 
are not of a size or shape to allow of our considering every 
part of them as moving in the same plane, they in reality re¬ 
volve round a line, which line is called the axis of motion. 
In a top, for instance, when spinning on its point, the axis is 
the line which passes through the middle of it, perpendicu¬ 
larly to the floor. 

Caroline. The axle of the flyers of the windmill, is then 
the axis of its motion; but is the centre of motion always in 
the middle of a body? 

Mrs. B . No, not always. The middle point of a body, 
is called its centre of magnitude, or position, that is, the 
centre of its mass or bulk. Bodies have also another centre, 
called the centre of gravity, which I shall explain to you; but 
at present we must confine ourselves to the axis of motion. 
This line you must observe remains at rest, whilst all the 
other parts of the body move around it; when you spin a top 
the axis is stationary whilst every other part is in motion 
round it. 

Caroline. But a top generally has a motion forwards be¬ 
sides its spinning motion; and then no point within it can 
be at rest? 


54 


ON COMPOUND MOTION. 


Mrs. B. What I say of the axis of motion, relates only 
to circular motion; that is to say, motion round a line, and 
not to that which a body may have at the same time in any 
other direction. There is one circumstance in circular mo¬ 
tion, which you must carefully attend to; which is, that the 
further any part of a body is from the axis of motion, the 
greater is its velocity; as you approach that line, the velocity 
of the parts gradually diminish till you reach the axis of mo¬ 
tion, which is perfectly at rest. 

Caroline. But, if every part of the same body did not 
move with the same velocity, that part which moved quick¬ 
est, must be separated from the rest of the body, and leave 
it behind? 

Mrs. B. You perplex yourself by confounding the idea 
of circular motion, with that of motion in a right line; you 
must think only of the motion of a body round a fixed line, 
and you will find, that if the parts farthest from the centre 
had not the greatest velocity, those parts would not be able 
to keep up with the rest of the body, and would be left be¬ 
hind. Do not the extremities of the vanes of a windmill 
move over a much greater space, than the parts nearest the 
axis of motion? (plate III. fig. 1.) The three dotted circles 
describe the paths in which three different parts of the vanes 
move, and though the circles are of different dimensions the 
vanes describe each of them in the same space of time. 

Caroline. Certainly they do; and I now only wonder, 
that we neither of us ever made the observation before: and 
the same effect must take place in a solid body, like the top 
in spinning; the most bulging part of the surface must move 
with the greatest rapidity. 

Mrs. B. The force which confines a body to a centre, 
round which it moves is called the centripetal force; and that 
force, which impels a body to fly from the centre, is called 
the centrifugal force; in circular motion these two forces 
constantly balance each other; otherwise the revolving body 
would either approach the centre or recede from it, accord¬ 
ing as the one or the other prevailed. 

Caroline. When I see any body moving in a circle, I 
shall remember, that it is acted on by two forces. 




IV A l l. lit. 


























































































ON COMPOUND MOTION. 


55 


Mrs. B. Motion, either in a circle, an ellipsis, or any 
other curve-line, must be the result of the action of two 
forces; for you know, that the impulse of one single force, 
always produces motion in a right line. 

Emily. And if any cause should destroy the centripetal 
force, the centrifugal force would alone impel the body, and 
it would I suppose fly off in a straight line from the centre 
to which it had been confined. 

Mrs. B. It would not fly off in a right line from the 
centre; but in a right line in the direction in which it was 
moving, at the instant of its release; if a stone, whirled round 
in a sling, gets loose at the point A, (plate III. fig. 2.) it 
flies off in the direction A B; this line is called a tangent , it 
touches the circumference of the circle, and forms a right 
angle with a line drawn from that point of the circumference 
to the centre of the circle C. 

Emily. You say, that motion in a curve-line, is owing 
to two forces acting upon a body; but when I throw this ball 
in a horizontal direction it describes a curve-line in falling; 
and yet it is only acted upon by the force of projection; there 
is no centripetal force to confine it, or produce compound 
motion. 

Mrs. B. A ball thus thrown, is acted upon by no less 
than three forces; the force of projection, which you com¬ 
municate to it; the resistance of the air through which it 
passes, which diminishes its velocity, without changing its 
direction; and the force of gravity, which finally brings it to 
the ground. The power of gravity, and the resistance of the 
air, being always greater than any force of projection we can 
give a body, the latter is gradually overcome, and the body 
brought to the ground; but the stronger the projectile force, 
the longer will these powers be in subduing it, and the fur¬ 
ther the body will go before it falls. 

Caroline. A shot fired from a cannon, for instance, will 
go much further, than a stone projected by the hand. 

Mrs. B. Bodies thus projected, you observe, describe a 
curve-line in their descent; can you account for that? 

Caroline. No; I do not understand, why it should not 
fall in the diagonal of a square. 


56 


ON COMPOUND MOTION. 


Mrs. B. You must consider that the force of projection 
is strongest when the ball is first thrown; this force, as it 
proceeds, being weakened by the continued resistance of the 
air, the stone, therefore, begins by moving in a horizontal 
direction; but as the stronger powers prevail, the direction 
of the ball will gradually change from a horizontal, to a 
perpendicular line. Projection alone, would drive the ball 
A, to B, (fig. 3.) gravity would bring it to C; therefore, 
when acted on in different directions, by these two forces, 
it moves between, gradually inclining more and more to the 
force of gravity, in proportion as this accumulates; instead 
therefore of reaching the ground at D, as you suppose it 
would, it fails somewhere about E. 

Caroline. It is precisely so; look, Emily, as I throw this 
ball directly upwards, how the resistance of the air and 
gravity conquers projection. Now I will throw it upwards 
obliquely: see, the force of projection enables it, for an in¬ 
stant, to act in opposition to that of gravity; but it is soon 
brought down again. 

Mrs. B. The curve-line which the ball has described, 
is called in geometry a parabola; but when the ball is thrown 
perpendicularly upwards, it will descend perpendicularly; 
because the force of projection, and that of gravity, are in 
the same line of direction. 

We have noticed the centres of magnitude, and of mo¬ 
tion; but I have not yet explained to you, what is meant by 
the centre of gravity; it is that point in a body, about which 
all the parts exactly balance each other; if therefore that 
point is supported, the body will not fall. Do you understand 
this? 

Emily. I think so; if the parts round about this point 
have an equal tendency to fall, they will be in equilibrium, 
and as long as this point is supported, the body can not fall. 

Mrs . B. Caroline, what would be the effect, were any 

other point of the body alone supported? 

Caroline. The surrounding parts no longer balancing 
each other, the body, I suppose, would fall on the side at 
which the parts are heaviest. 

Mrs. B. Infallibly; whenever the centre of gravity is 


ON COMPOUND MOTION. 


57 


unsupported, the body must fall. This sometimes happens 
with an overloaded wagon winding up a steep hill, one 
side of the road being more elevated than the other; let us 
suppose it to slope as is described in this figure, (plate III. 
fig. 4.) we will say, that the centre of gravity of this loaded 
wagon is at the point A. Now your eye will tell you, that 
a wagon thus situated, will overset; and the reason is, that 
the centre of gravity A, is not supported; for if you draw a 
perpendicular line from it to the ground at C, it does not fall 
under the wagon within the wheels, and is therefore not 
supported by them. 

Caroline. I understand that perfectly; but what is the 
meaning of the other point B? 

Mrs. B. Let us, in imagination take off the upper part 
of the load; the centre of gravity will then change its situa¬ 
tion, and descend to B, as that will now be the point about 
which the parts of the less heavily laden wagon will balance 
each other. Will the wagon now be upset? 

Caroline. No, because a perpendicular line from that 
point falls within the wheels at D, and is supported by them; 
and when the centre of gravity is supported, the body will 
not fall. 

Emily. Yet I should not much like to pass a wagon in 
that situation, for, as you see, the point D is but just with¬ 
in the left wheel; if the right wheel was merely raised, by 
passing over a stone, the point D would be thrown on the 
outside of the left wheel, and the wagon would upset. 

Caroline. A wagon, or any carriage whatever, will then 
be most firmly supported, when the centre of gravity falls 
exactly between the wheels; and that is the case in a level 
road. 

Pray, whereabouts is the centre of gravity of the human 
body? 

Mrs . B. Between the hips; and as long as we stand up¬ 
right, this point is supported by the feet; if you lean on one 
side, you will find that you no longer stand firm. A rope- 
dancer performs all his feats of agility, by dexterously sup¬ 
porting his centre of gravity; whenever he finds that he is 
in danger of losing his balance, he shifts the heavy pole, 


ON COMPOUND MOTION. 


*8 

which he holds in his hands, in order to throw the weight 
towards the side that is deficient, and thus by changing the 
situation of the centre of gravity, he restores his equilibrium, 

Caroline. When a stick is poised on the tip of the finger, 
is it not by supporting its centre of gravity? 

Mrs. B. Yes; and it is because the centre of gravity is 
not supported, that spherical bodies roll down a slope. A 
sphere being perfectly round, can touch the slope but by a 
single point, and that point can not be perpendicularly un¬ 
der the centre of gravity, and therefore can not be support¬ 
ed, as you will perceive by .examining this figure, (fig. 5. 
plate III.) 

Emily. So it appears; yet I have seen a cylinder of wool 
roll up a slope; how is that contrived? 

Mrs. B. It is done by plugging one side of the cylinder 
with lead, as at B, (fig. 6. plate III.) the body being no 
longer of a uniform density, the centre of gravity is removed 
from the middle of the body to some point in the lead, as 
that substance is much heavier than wood; now you may 
observe that in order that the cylinder may roll down the 
plane, as it is here situated, the centre of gravity must rise, 
which is impossible; the centre of gravity must always de¬ 
scend in moving, and will descend by the nearest and rea¬ 
diest means, which will be by forcing the cylinder up the 
slope, until the centre of gravity is supported, and then it 
stops. 

Caroline. The centre of gravity, therefore, is not always 
in the middle of a body. 

Mrs. B. No, that point we have called the centre of 
magnitude; when the body is of an uniform density the 
centre of gravity is in the same point; but when one part 
of the body is composed of heavier materials than another 
part, the centre of gravity being the centre of the weight of 
the body, can no longer correspond with the centre of mag¬ 
nitude. Thus you see the centre of gravity of this cylinder 
plugged with lead, can not be in the same spot as the centre 
of magnitude. 

Emily. Bodies, therefore, consisting but of one kind of 
substance, as wood, stone, or lead, and whose densities are 


ON COMPOUND MOTION. 


59 


consequently uniform, must stand more firmly, and be more 
difficult to overset, than bodies composed of a variety of 
substances, of different densities, which may throw the cen¬ 
tre of gravity on one side. 

Mrs. B. Yes, but there is another circumstance which 
more materially affects the firmness of their position, and 
that is their form. Bodies that have a narrow base are ea¬ 
sily upset, for if they are the least inclined, their centre is 
no longer supported, as you may perceive in fig. 6. 

Caroline. I have often observed with what difficulty a 
person carries a single pail of water; it is owing, I suppose, 
to the centre of gravity being thrown on one side, and the 
opposite arm is stretched out to endeavour to bring it back 
to its original situation; but a pail hanging to each arm is 
carried without difficulty, because they balance each other, 
and the centre of gravity remains supported by the feet. 

Mrs. B. Very well; I have but one more remark to make 
on the centre of gravity, which is, that when two bodies 
are fastened together by a line, string, chain, or any power 
whatever, they are to be considered as forming but one 
body; if the two bodies be of equal weight, the centre of 
gravity will be in the middle of the line which unites them, 
(fig. 7.) but if one be heavier than the other, the centre of 
gravity will be proportionally nearer the heavy body than 
the light one. (fig. 8.) If you were to carry a rod or pole 
with an equal weight fastened at each end of it, you would 
hold it in the middle of the rod, in order that the weights 
should balance each other; whilst if it bad unequal weights 
at each end you would hold it nearest the greater weight, 
to make them balance each other. 

Emily. And in both cases we should support the centre 
of gravity; and if one weight be very considerably larger 
than the other, the centre of gravity will be thrown out of 
the rod into the heaviest weight, (fig. 9.) 

Mrs. B. Undoubtedly. 


CONVERSATION V. 


ON THE MECHANICAL POWERS. 


OF THE POWER OF MACHINES.—OF THE LEVER IN GENERAL.-OF THE LEVER 

OF THE FIRST KIND, HAVING THE FULCRUM BETWEEN THB POWER AND 

THE WEIGHT.-OF THE LEVER OF THE SECOND KIND, HAVING THE WEIGHT 

BETWEEN THE POWER AND THE FULCRUM.—OF THE LEVER OF THE THIRD 
KIND, HAVING THE POWER BETWEEN THE FULCRUM AND THE WEIGHT. 


MRS. B. 

We may now proceed to examine the mechanical pow¬ 
ers; they are six in number, one or more of which enters 
into the composition of every machine. The lever , the 
pulley , the wheel and axle , the inclined plane , the wedge, and 
the screw . 

In order to understand the power of a machine, there are 
four things to be considered. 1st. The power that acts: 
this consists in the effort of men or horses, of weights, 
springs, steam, &c. 

2dly. The resistance which is to be overcome by the 
power; this is generally a weight to be moved. The power 
must always be superior to the resistance, otherwise the 
machine could not be put in motion. 

Caroline. If for instance the resistance of a carriage was 
greater than the strength of the horses employed to draw it, 
they would not be able to make it move. 

Mrs. B. 3dly. We are to consider the centre of motion, 
or as it is termed in mechanics, the fulcrum; this you may 
recollect is the point about which all the parts of the body 
move; and lastly, the respective velocities of the power, 
and of the resistance. 









































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P.LATTE IV . 
















































































ON THE MECHANICAL POWERS. • 


61 


Emily. That must depend upon their respective distances 
from the axis of motion; as we observed in the motion of 
the vanes of the windmill. 

Mrs. B. We shall now examine the power of the lever. 
The lever is an inflexible rod or beam of any kind, that is 
to say, one which will not bend in any direction. For in¬ 
stance, the steel rod to which these scales are suspended is 
a lever, and the point in which it is supported the fulcrum, 
or centre of motion; now, can you tell me why the two 
scales are in equilibrium? 

Caroline. Being both empty, and of the same weight, 
they balance each other. 

Emily. Or, more correctly speaking, because the centre 
of gravity common to both is supported. 

Mrs. B. Very well; and which is the centre of gravity 
of this pair of scales? (fig. 1. plate IV.) 

Emily. You have told us that when two bodies of equal 
weight were fastened together, the centre of gravity was in 
the middle of the line that connected them; the centre of 
gravity of the scales must therefore be in the fulcrum F of 
the lever which unites the two scales; and corresponds with 
the centre of motion. 

Caroline. But if the scales contain different weights, the 
centre of gravity would no longer be in the fulcrum of the 
lever, but remove towards that scale which contained the 
heaviest weight; and since that point would no longer be 
supported, the heavy scale would descend and out-weigh 
the other. 

Mrs. B. True; but tell me, can you imagine any mode 
by which bodies of different weights can be made to balance 
each other, either in a pair of scales, or simply suspended 
to the extremities of the lever? for the scales are not an es¬ 
sential part of the machine, they have no mechanical power, 
and are used merely for the convenience of containing the 
substance to be weighed. 

Caroline. What! make a light body balance a heavy one? 
I can not conceive that possible, 

Mrs. B. The fulcrum of this pair of scales (fig. 2.) is 
moveable, you see; I can take it off the prop, and fasten it 
6 * 


(12 ON THE MECHANICAL POWERS. 

on again in another part; this part is now become the ful¬ 
crum, but it is no longer in the centre of the lever. 

Caroline. And the scales are no longer true; for that 
which hangs on the longest side of the lever descends. 

Mrs. B. The two parts of the lever divided by the 
fulcrum are called its arms, you should therefore say the 
longest arm, not the longest side of the lever. These arms 
are likewise frequently distinguished by the appellations of 
the acting and the resisting part of the lever. 

Your observation is true that the balance is now destroyed; 
but it will answer the purpose of enabling you to compre¬ 
hend the power of a lever when the fulcrum is not in the 
centre. 

Emily. This would be an excellent contrivance for those 
who cheat in the weight of their goods; by making the ful¬ 
crum a little on one. side, and placing the goods in the scale 
which is suspended to the longest arm of the lever, they 
w r ould appear to weigh more than they do in reality. 

Mrs. B. You do not consider how easily the fraud would 
bj detected; for on the scales being emptied they would not 
hang in equilibrium. 

Emily. True; I did not think of that circumstance. But 
I do not understand why the longest arm of the lever should 
not be in equilibrium with the other? 

Caroline. It is because it is heavier than the shortest 
arm; the centre of gravity, therefore, is no longer supported. 

Mrs. B. You are right, the fulcrum is no longer in the 
centre of gravity; but if we can contrive to make the ful¬ 
crum in its present situation become the centre of gravity, 
the scales will again balance each other; for you recollect 
that the centre of gravity is that point about which every 
part of the body is in equilibrium. 

Emily. It has just occurred to me how this may be ac¬ 
complished; put a great weight into the scale suspended to 
the shortest arm of the lever, and a smaller one into that 
suspended to the longest arm. Yes, I have discovered it— 
look Mrs. B., the scale on the shortest arm will carry 21bs., 
and that on the longest arm only one, to restore the balance. 
( fi g- s ■) 


ON THE MECHANICAL POWERS. 


63 


Mrs. B. You see, therefore, that it is not so impractica¬ 
ble as you imagined, to make a heavy body balance a light 
one; and this is in fact the means by which you thought an 
imposition in the weight of goods ’might be effected, as a 
weight of ten or twelve ounces might thus be made to ba¬ 
lance a pound of goods. Let us now take off the scales that 
we may consider the lever simply; and in this state you sec 
that the fulcrum is no longer the centre of gravity; but it is, 
and must ever be, the centre of motion, as it is the only 
point which remains at rest, while the other parts move 
about it. 

Caroline. It now resembles the two opposite vanes of a 
windmill, and the fulcrum the point round which they move. 

Mrs. B. In describing the motion of those vanes, you 
may recollect our observing that the farther a body is from 
the axis of motion the greater is its velocity. 

Caroline. That I remember and understood perfectly. 

Mrs. B. You comprehend then, that the extremity of the 
longest arm of a lever must move with greater velocity than 
that of the shortest arm? 

Emily. No doubt, because it is farthest from the centre 
of motion. And pray, Mrs. B., when my brothers play at 
see-saw, is not the plank on which they rode a kind of lever? 

Mrs. B. Certainly; the log of wood which supports it 
from the ground is the fulcrum, and those who ride repre¬ 
sent the power and the resistance at each end of the lever. 
And have you not observed that when those who ride are of 
equal weight, the plank must be supported in the middle to 
make the two arms equal; whilst if the persons differ in 
weight, the plank must be drawn a little farther over the 
prop, to make the arms unequal, and the lightest person who 
represents the resistance, must be placed at the extremity 
of the longest arm. 

Caroline. That is always the case when I ride on a plank 
with my youngest brother; I have observed also that the 
lightest person has the best ride, as he moves both further 
and quicker; and I now understand that it is because he is 
more distant from the centre of motion. 

Mrs. B. The greater velocity with which your little 
brother moves, renders his momentum equal to yours. 


64 ON THE MECHANICAL POWERS. 

Caroline. Yes; I have the most gravity, he the greatest 
velocity; so that upon the whole our momentums are equal.— 
But you said, Mrs. B., that the power should be greater than 
the resistance to put the machine in motion; how then can 
the plank move if the momentums of the persons who ride 
are equal? 

Mrs. B. Because each person at his descent touches the 
ground with his feet; the reaction of which gives him an 
impulse which increases his velocity; this spring is requisite 
to destroy the equilibrium of the power and the resistance, 
otherwise the plank would not move. Did you ever observe 
that a lever describes the arc of a circle in its motion? 

Emily. No; it appears to me to rise and descend per¬ 
pendicularly; at least I always thought so. 

Mrs. B. I believe I must make a sketch of you and your 
brother riding on a plank, in order to convince you of your 
error, (fig. 4. plate IY.) You may now observe that a lever 
can move only round the fulcrum, since that is the centre of 
motion; it would be impossible for you to rise perpendicu¬ 
larly to the point A, or for your brother to descend in a 
straight line to the point B; you must in rising and he in 
descending describe arcs of your respective circles. This 
drawing shows you also how much superior his velocity must 
be to yours; for if you could swing quite round, you would 
each complete your respective circles in the same time. 

Caroline. My brother’s circle being much the largest, he 
must undoubtedly move the quickest. 

Mrs. B. Now tell me, do you think that your brother 
could raise you as easily without the aid of a lever? 

Caroline. Oh no, he could not lift me off the ground. 

Mrs. B. Then I think you require no further proof of 
the power of a lever, since you see what it enables your 
brother to perform. 

Caroline. I now understand what you meant by saying, 
that in mechanics, motion is opposed to matter, for it is my 
brother’s velocity which overcomes my weight. 

Mrs. B. You may easily imagine, what enormous weights 
may be raised by levers of this description, for the longer 
the acting part of the lever in comparison to the resisting 


ON THE MECHANICAL POWERS. 


65 


part, the greater is the effect produced by it; because the 
greater is the velocity of the power compared to that of the 
weight. 

There are three different kinds of levers; in the first the 
fulcrum is between the power and the weight. 

Caroline . This kind then comprehends the several levers 
you have described. 

Mrs. B. Yes, when in levers of the first kind, the ful¬ 
crum is equally between the power and the weight, as in the 
balance, the power must be greater than the weight, in or¬ 
der to move it; for nothing can in this case be gained by ve¬ 
locity; the two arms of the lever being equal, the velocity of 
their extremities must be so likewise. The balance is there¬ 
fore of no assistance as a mechanical power, but it is ex¬ 
tremely useful to estimate the respective weights of bodies. 

But when (fig. 5.) the fulcrum F of a lever is not equally 
distant from the power and the weight, and that the power 
P acts at the extremity of the longest arm, it may be less than 
the weight W, its deficiency being compensated by its supe¬ 
rior velocity; as we observed in the see-saw. 

Emily. Then when we want to lift a great weight, we 
must fasten it tn the shortest arm of a lever, and apply our 
strength to the longest arm? 

Mrs. B. If the case will admit of your putting the end 
of the lever under the weight, no fastening will be required; 
as you will perceive by stirring the fire. 

Emily. Oh yes! the poker is a lever of the first kind, 
the point where it rests against the bars of the grate whilst 
I am stirring the fire, is the fulcrum; the short arm or re¬ 
sisting part of the lever, is employed in lifting the weight, 
which is the coals, and my hand is the power applied to the 
longest arm, or acting part of the lever. 

Mrs. B. Let me hear, Caroline, whether you can equally 
well explain this instrument, which is composed of two 
levers united in one common fulcrum. 

Caroline. A pair of scissors! 

Mrs. B. You are surprised, but if you examine their 
construction, you will discover that it is the power of the 
lever that assists us in cutting with scissors. 


r 


66 


ON THE MECHANICAL POWERS. 


Caroline. Yes; I now perceive that the point at which 
the two levers are screwed together, is the fulcrum; the 
handles, to which the power of the fingers is applied, are 
the extremities of the acting part of the levers, and the cut¬ 
ting part of the scissors, are the resisting parts of the levers: 
therefore, the longer the handles and the shorter the points 
of the scissors, the more easily you cut with them. 

Emily, That I have often observed, for when I cut paste¬ 
board or any hard substance, I always make use of that part 
of the scissors nearest the screw or rivet, and I now under¬ 
stand why it increases the power of cutting; but I confess 
that I never should have discovered scissors to have been 
double levers; and pray are not snuffers levers of a similar 
description ? 

Mrs. B. Yes, and most kinds of pincers; the great power 
of which consists in the resisting part of the lever being very 
short in comparison with the acting part. 

Caroline. And of what nature are the two other kinds of 
levers? 

Mrs. B. In levers of the second kind, the weight, instead 
of being at one end, is situated between the power and the 
fulcrum, (fig. 6.) 

Caroline. The weight and the fulcrum have here 
changed .places; and what advantage is gained by this kind 
of lever? 

Mrs. B. In moving it, the velocity of the power must 
necessarily be greater than that of the weight, as it is more 
distant from the centre of the motion. Have you ever seen 
your brother move a snow-ball by means of a strong stick, 
when it became too heavy for him to move without assistance? 

Caroline. Oh yes; and this was a lever of the second 
order; (fig. 7.) the end of the stick, which he thrusts under 
the ball, and which rests on the ground, becomes the ful¬ 
crum: the ball is the weight to be moved, and the power his 
hands, applied to the other end of the lever. In this in¬ 
stance there is an immense difference in the length of the 
arms of the lever; for the weight is almost close to the ful¬ 
crum. 

Mrs. B. And the advantage gained is proportional to 


ON THE MECHANICAL POWERS'. 


67 


this difference. Fishermen’s boats are by levers of this de¬ 
scription raised from the ground to be launched into the sea, 
by means of slippery pieces of board which are thrust under 
the keel. The most common example that we have of le¬ 
vers of the second kind is in the doors of our apartments. 

Emily. The hinges represent the fulcrum, our hands the 
power applied to the other end of the lever; but where is 
the weight to be moved? 

Mrs. B. The door is the weight, and it consequently 
occupies the whole of the space between the power and the 
fulcrum. Nut crackers are double levers of this kind: the 
hinge is the fulcrum, the nut the resistance, and the hands 
the power. 

In levers of the third kind (fig. 8.) the fulcrum is again 
at one of the extremities, the weight or resistance at the 
other, and it is now the power which is applied between 
the fulcrum and the. resistance. 

Emily. The fulcrum, the weight and the power, then, 
each in their turn, occupy some part of the middle of the le¬ 
ver between its extremities. But in this third kind of lever, 
the weight being farther from the centre of motion than the 
power, the difficulty of raising it seems increased rather 
than diminished. 

Mrs . B. That is very true; a lever of this kind is there¬ 
fore never used, unless absolutely necessary, as is the case 
in lifting up a ladder perpendicularly in order to place it 
against a wall; the man who raises it can not place his 
hands on the upper part of the ladder, the power, therefore, 
is necessarily placed much nearer the fulcrum than the 
weight. 

Caroline. Yes, the hands are the power, the ground the 
fulcrum, and the upper part of the ladder the weight. 

Mrs. B. Nature employs this kind of lever in the struc¬ 
ture of the human frame. In lifting a weight with the hand, 
the lower part of the arm becomes a lever of the third kind; 
the elbow is the fulcrum, the muscles of the fleshy part of 
the arm the power; and as these are nearer to the elbow 
than the hand, it is necessary that their power should ex¬ 
ceed the weight to be raised. 


68 


ON THE MECHANICAL POWERS. 


Emily. Is it not surprising that nature should have fur¬ 
nished us with such disadvantageous levers? 

Mrs B . The disadvantage, in respect to power is more 
than counterbalanced by the convenience resulting from 
this structure of the arm; and it is no doubt that which is 
best adapted to enable it to perform its various functions. 

We have dwelt so long on the lever, that we must re¬ 
serve the examination of the other mechanical powers to our 
next interview. 



\ 





* 












Pirate v. 








































































































CONVERSATION V. 


CONTINUED. 


ON THE MECHANICAL POWERS. 


OF THE PVLLET.-OP THE WHEEL AND AXLE.—OF THE INCLINED PLANE.- 

OF THE WEDGE.—OF THE SCREW. 

MRS. B. 

The pulley is the second mechanical power we are to 
examine. You both, I suppose, have seen a pulley? 

Caroline. Yes, frequently: it is a circular and ilat piece 
of wood or metal, with a string which runs in a groove round 
it; by means of which, a weight may be pulled up; thus 
pullies are used for drawing up curtains. 

Mrs. B. Yes; but in that instance the pullies are fixed, 
and do not increase the power to raise the weights, as you 
will perceive by this figure, (plate Y. fig. 1.) Observe that 
the fixed pulley is on the same principal as the lever of a 
pair of scales, in which the fulcrum F being in the centre 
of gravity, the power P and the weight W, are equally dis- 
I tant from it, and no advantage is gained. 

Emily. Certainly; if P represents the power employed 
ji to raise the*weight W, the power must be greater than the 

i weight in order to move it. But of what use then are pul- 
, lies in mechanics? 

Mrs. B. The next figure represents a pully which is not 
" fixed, (fig. 2.) and thus situated you will perceive that it 

ii affords us mechanical assistance. In order to raise the 

7 



70 


ON THE MECHANICAL POWERS. 


weight W, one inch, P, the power, must draw the strings 
B and C,one inch each; the whole string is therefore short¬ 
ened two inches, while the weight is raised only one. 

Emily. That I understand: if P drew the string but one 
inch, the weight would be raised only half an inch, because 
it would shorten the strings B and C half an inch each, and 
consequently the pulley with the weight attached to it, can 
be raised only half an inch. 

Caroline. I am ashamed of my stupidity; but I confess 
that I do not understand this; it appears to me that the 
weight would be raised as much as the string is shortened 
by the power. 

Mrs. B. I will endeavour to explain it more clearly. I 
fasten this string to a chair and draw it towards me; I have 
now shortened the string, by the act of drawing it, one yard. 

Caroline. And the chair, as I suppose, has advanced 
one yard. 

Mr6. B. This exemplifies the nature of a single fixed 
pulley only. Now unfasten ■ the string, and replace the 
chair where it stood before. In order to represent the move- 
able pulley, we must draw the chair forwards by putting 
the string round it; one end of the string may be fastened to 
the leg of the table, and I shall draw the chair by the other 
end of the string. I have again shortened the string one 
yard; how much has the chair advanced? 

Caroline. I now understand it; the chair represents the 
weight to which the moveable pulley is attached; and it is 
very clear that the weight can be drawn only half the length 
you draw the string. I believe the circumstance that per¬ 
plexed me was, that I did not observe the difference that 
results from the weight being attached to the pulley, instead 
of being fastened to the string, as is the case in the fixed 
pulley. 

Emily. But I do not yet understand the advantage of 
pullies; they seem to me to increase rather than diminish 
the difficulty of raising weights, since you must draw the 
string double the length that you raise the weight; whilst 
with a single pulley, or without any pulley, the weight is 
raised as much as the string is shortened. 


ON THE MECHANICAL POWERS. 


71 


Mrs. B. The advantage of a moveable pulley consists 
in dividing the difficulty; we must draw, it is true, twice 
the length of the string, but then only half the strength is 
required that would be necessary to raise the weight with¬ 
out the assistance of a moveable pulley. 

Emily. So that the difficulty is overcome in the same 
manner as it would be, by dividing the weight into two 
equal parts, and raising them successively. 

t Mrs. B. Exactly. You must observe, that with a move- 
able pulley the velocity of the power is double that of the 
weight, since the power P (fig. 2.) moves two inches whilst 
the weight W moves one inch; therefore the power need 
not be more than half the weight to make their momentums 
equal. 

Caroline. Pulleys act then on the same principle as the 
lever, the deficiency of strength of the power being com¬ 
pensated by its superior velocity. 

Mrs. B. You will find, that all mechanical power is 
founded on the same principle. 

Emily. But may it not be objected to pulleys, that a 
longer time is required to raise a weight by their aid than 
without it; for what you gain in power you lose in time? 

Mrs. B. That, my dear, is the fundamental law in me¬ 
chanics: it is the case with the lever as well as the pulley; 
and you will find it to be so with all the other mechanical 
powers. 

Caroline. I do not see any advantage in the mechani¬ 
cal powers then, if what we gain by them one way is lost 
another. 

Mrs. B. Since we are not able to increase our natural 
strength, is not that science of wonderful utility, by means 
of which we may reduce the resistance or weight of any 
body to the level of our strength? This the mechanical 
powers enable us to accomplish, by dividing the resistance 
of a body into parts which we can successively overcome. 
It is true, as you observe, that it requires a sacrifice of time 
to attain this end, but you must be sensible how very ad¬ 
vantageously it is exchanged for power: the utmost exertion 
we can make adds but little to our natural strength, whilst 



12 


ON THE MECHANICAL POWERS, 


ive have a much more unlimited command of time. You 
can now understand, that the greater the number of pulleys 
connected by a string, the more easily the weight is raised, 
as the difficulty is divided amongst the number of strings, 
or rather of parts into which the string is divided by the 
pulleys. Several pulleys thus connected, form what is' called 
a system, or tackle of pulleys, (fig. 3.) You may have seen 
them suspended from cranes to raise goods into warehouses, 
and in ships to draw up the sails. 

Emily . But since a fixed pulley affords us no mechanical 
aid, why is it ever used? 

Mrs. B. Though it does not increase our power, it is 
frequently useful for altering its direction. A single pulley 
enables us to draw up a curtain, by drawing down the string 
connected with it; and we should be much at a loss to ac¬ 
complish this simple operation without its assistance. 

Caroline . There would certainly be some difficulty in 
ascending to the bead of the curtain, in order to draw it up. 
Indeed, I now recollect having seen workmen raise small 
weights by this means, which seemed to answer a very use¬ 
ful purpose. 

Mrs. B. In shipping, both the advantages of an increase 
of power and a change of direction, by means of pulleys, 
are united: for the sails are raised up the masts by the sailors 
on deck, from the change of direction which the pulley ef¬ 
fects, and the labour is facilitated by the mechanical power 
of a combination of pulleys. 

Emily. But the pulleys on ship-board do not appear to 
me to be united in the manner you have shown us. 

Mrs. B. They are, I believe, generally connected as 
described in figure 4, both for nautical, and a variety of 
other purposes; but in whatever manner pulleys are con¬ 
nected by a single string, tjie mechanical power is the same. 

The third mechanical power is the wheel and axle. Let 
us suppose (plate VI. fig. 5.) the weight W. to be a bucket 
of water in a well, which we raise by winding the rope, to 
which it is attached, round the axle; if this be done without 
a wheel to turn the axle, no mechanical assistance is re¬ 
ceived. The axle without a wheel is as impotent as a single 


ON THE MECHANICAL POWERS. 


73 


fixed pulley, or a lever, whose fulcrum is in the centre: but 
add the wheel to the axle, and you will immediately find 
the bucket is raised with much less difficulty. The velocity 
of the circumference of the wheel is as much greater than 
that of the axle, as it is further from the centre of motion; 
for the wheel describes a great circle in the same space of 
time that the axle describes a small one, therefore the power 
is increased in the same proportion as the circumference of 
the wheel is greater than that of the axle. If the velocity of 
the wheel is twelve times greater than that of the axle, a 
power nearly twelve times less than the weight of the bucket 
would be able to raise it. 

Emily. The axle acts the part of the shorter arm of the 
lever, the wheel that of the longer arm. 

Caroline. In raising water, there is commonly, I believe, 
instead of a wheel attached to the axle, only a crooked 
handle, which answers the purpose of winding the rope round 
the axle, and thus raising the bucket. 

Mrs. B. In this manner (fig. 6.;) now if you observe the 
dotted circle which the handle describes in winding up the 
rope, you will perceive that the branch of the handle A, 
which is united to the axle, represents the spoke of a wheel, 
and answers the purpose of an entire wheel; the other branch 
B affords no mechanical aid, merely serving as a handle to 
turn the wheel. 

Wheels are a very essential part of most machines: they 
are employed in various ways; but, when fixed to the axle, 
their mechanical power is always the same: that is, as the 
circumference of the wheel exceeds that of the axle, so 
much will the energy of the power be increased. 

Caroline. Then the larger the wheel the greater must 
be its effect? 

Mrs. B. Certainly. If you have ever seen any considera» 
bie mills or manufactures, you must have admired the im¬ 
mense wheel, the revolution of which puts the whole of the 
machinery into motion; and though so great an effect is pro¬ 
duced by it, a horse or two has sufficient power to turn it; 
sometimes a stream of water is used for that purpose, but of 
late years, a steam-engine has been found both the most 


74 


ON THE MECHANICAL POWERS. 


powerful and the most convenient mode of turning the 
wheel. 

Caroline. Do not the vanes of a windmill represent a 
wheel, Mrs. B.? 

Mrs. B. Yes; and in this instance we have the advan¬ 
tage of a gratuitous force, the wind, to turn the wheel. One 
of the great benefits resulting from the use of machinery is, 
that it gives us a sort of empire over the powers of nature, 
and enables us to make them perform the labour which 
would otherwise fall to the lot of man. When a current of 
wind, a stream of water, or the expansive force of stream, 
performs our task, we have only to superintend and regulate 
their operations. 

The fourth mechanical power is the inclined plane; this 
is nothing more than a slope, or declivity, frequently used to 
facilitate the drawing up of weights. It is not difficult to 
understand, that a weight may much more easily be drawn 
up a slope than it can be raised the same height perpendicu¬ 
larly. But in this, as well as the other mechanical powers, 
the facility is purchased by a loss of time (fig. 7.;) for the 
weight, instead of moving directly from A to C, must move 
from B to C, and as the length of the plane is to its height, 
so much is the resistance of the weight diminished. 

Emily. Yes; for the resistance, instead of being confined 
to the short line A C, is spread over the long line B C. 

Mrs. B. The wedge, which is the next mechanical 
power, is composed of two inclined planes (fig. 8.:) you may 
have seen wood-cutters use it to cleave wood. The resistance 
consists in the cohesive attraction of the wood, or any other 
body which the wedge is employed to separate; and the ad¬ 
vantage gained by this power is in the proportion of half its 
width to its length; for while the wedge forces asunder the 
coherent particles of the wood to A and B, it penetrates 
downwards as far as C. 

Emily. The wedge, then, is rather a compound than a 
distinct mechanical power, since it is composed of two in¬ 
clined planes. 

Mrs. B. It is so. All cutting instruments are constructed 
upon the principle of the inclined plane, or the wedge: those 


ON THE MECHANICAL POWERS. 75 

that have but one edge sloped, like the chisel, may be re¬ 
ferred to the inclined plane; whilst the axe, the hatchet, 
and the knife (when used to split asunder) are used as 
wedges. 

Caroline. But a knife cuts best when it is drawn across 
the substance it is to divide. We use it thus in cutting meat, 
we do not chop it to pieces. 

Mrs. B . The reason of this is, that the edge of a knife 
is really a very fine saw, and therefore acts best when used 
like that instrument. 

The screw, which is the last mechanical power, is more 
complicated than the others. You will see by this figure, 
(fig. 9.) that it is composed of two parts, the screw and the 
nut. The screw S is a cylinder, with a spiral protuberance 
coiled round it, called the thread; the nut N is perforated 
to contain the screw, and the inside of the nut has a spiral 
groove made to fit the spiral thread of the screw. 

Caroline . It is just like this little box, the lid of which 
screws on the box as you have described; but what is this 
handle which projects from the nut? 

Mrs. B. It is a lever, which is attached to the nut, with¬ 
out which the screw is never used as a mechanical power: 
the nut with a lever L attached to it, is commonly called a 
winch. The power of the screw, complicated as it appears, 
is referable to one of the most simple of the mechanical 
powers; which of them do you think it is? 

Caroline. In appearance, it most resembles the wheel 
and axle. 

Mrs. B. The lever, it is true, has the effect of a wheel, 
as it is the means by which you wind the nut round; but the 
lever is not considered as composing a part of the screw, 
though it is true, that it is necessarily attached to it. But 
observe, that the lever, considered as a wheel, is not fast¬ 
ened to the axle or screw, but moves round it, and in so 
doing, the nut either rises or descends, according to the way 
in which you turn it. 

Emily. The spiral thread of the screw resembles, I 
think, an inclined plane: it is a sort of slope, by means of 



78 ON THE MECHANICAL POWERS. 

which the nut ascends more easily than it would do if raised 
perpendicularly; and it serves to support it when at rest. 

Mrs. B. Very well: if you cut a slip of paper in the 
form of an inclined plane, and wind it round your pencil, 
which will represent the cylinder, you will find that it makes 
a spiral line, corresponding to the spiral protuberance of the 
screw, (fig. 10.) 

Emily. Very true; the nut then ascends an inclined plane, 
but ascends it in a spiral, instead of a straight line: the closer 
the thread of the screw, the more easy the ascent; it is like 
having shallow, instead of steep steps to ascend. 

Mrs. B. Yes; excepting that the nut takes no steps, it 
gradually winds up or down; then observe, that the closer 
the threads of the screw, the greater the number of revolu¬ 
tions the winch must make; so that we return to the old 
principle,—what is saved in power is lost in time. 

Emily. Can not the power of the screw be increased 
also, by lengthening the lever attached to the nut? 

Mrs. B. Certainly. The screw, with the addition of 
the lever, forms a very powerful machine, employed either 
for compression or to raise heavy weights. It is used by 
book-binders, to press the leaves of books together; it is 
used also in cyder and wine presses, in coining, and for a 
variety of other purposes. 

All machines are composed of one or more of these six 
mechanical powers we have examined; I have but one more 
remark to make to you relative to them, which is, that fric¬ 
tion in a considerable degree diminishes their force, allow¬ 
ance must therefore always be made for ii, in the construc¬ 
tion of machinery. 

Caroline. By friction, do you mean one part of the ma¬ 
chine rubbing against another part contiguous to it? 

Mrs. B. Yes; friction is the resistance which bodies 
meet with in rubbing against each other; there is no such 
thing as perfect smoothness or evenness in nature; polished 
metals, though they wear that appearance more than any 
other bodies, are far from really possessing it; and their in¬ 
equalities may frequently be perceived through a good mag¬ 
nifying glass. When, therefore, the surfaces of the two 


ON THE MECHANICAL POWERS. 77 

bodies come in contact, the prominent parts of the one will 
often fall into the hollow parts of the other, and occasion 
more or less resistance to motion. 

Caroline. But if a machine is made of polished metal, as 
awatclffor instance, the friction must be very trifling? 

Mrs. B. In proportion as the surfaces of bodies are well 
polished, the friction is doubtless diminished; but it is al¬ 
ways considerable, and it is usually computed to destroy 
one third of the power of a machine. Oil or grease is used 
to lessen friction: it acts as a polish by filling up the cavi¬ 
ties of the rubbing surfaces, and thus making them slide 
more easily over each other. 

Caroline. Is it for this reason that wheels are greased, 
and the locks and hinges of doors oiled? 

Mrs. B . Yes; in these instances the contact of the rub¬ 
bing surfaces is so close, and the rubbing so continual, that 
notwithstanding their being polished and oiled, a consider¬ 
able degree of friction is produced. 

, There are two kinds of friction; the one occasioned by 
the sliding of the flat surface of a body, the other by the 
rolling of a circular body; the friction resulting from the 
first is much the most considerable, for great force is re¬ 
quired to enable the sliding body to overcome the ”esistance 
which the asperities of the surfaces in contact oppose to its 
motion, and it must be either lifted over, or break through 
them; whilst, in the other kind of friction, the rough parts 
roll over each other with comparative facility; hence it is, 
that wheels are often used for the sole purpose of diminish¬ 
ing the resistance of friction. 

Emily. This is one of the advantages of carriage-wheels, 
is it not? 

Mrs. B. Yes; and the larger the circumference of the 
wheel the more readily it can overcome any considerable 
obstacles, such as stones, or inequalities in the road. When, 
in descending a steep hill, we fasten one of the wheels, 
we decrease the velocity of the carriage, by increasing the 
friction. 

Caroline. That is to say, by converting the rolling fric¬ 
tion into the dragging friction. And when you had casters 


78 ON THE MECHANICAL POWERS. 

put to the legs of the table, in order to move it more easily, 
you changed the dragging into the rolling friction. 

Mrs. B. There is another circumstance which we have 
already noticed, as diminishing the motion of bodies, and 
which greatly affects the power of machines. ThfS is the 
resistance of the medium, in which a machine is worked. 
All fluids, whether of the nature of air or of water, are 
called mediums; and their resistance is proportioned to their 
density; for the more matter a body contains, the greater 
the resistance it will oppose to the motion of another body 
striking against it. 

Emily . It would then be much more difficult to work a 
machine under water than in the air? 

Mrs. B. Certainly, if a machine could be worked in 
vacao, and without friction, it would be perfect; but this is 
unattainable; a considerable reduction of power must there¬ 
fore be allowed for the resistance of the air. 

We shall here conclude our observations on the mechan¬ 
ical powers. At our next meeting I shall endeavour to give 
you an explanation of the motion of the heavenly bodies. 


CONVERSATION VI. 


CAUSES OP THE EARTH’S ANNUAL MOTION. 


OF THE PLANETS AND THEIR MOTION.—OF THE DIURNAL MOTION OF THE 
EARTH AND PLANETS. 


CAROLINE. 

I am come to you to-day quite elated with the spirit of 
opposition, Mrs. B.; for I have discovered such a powerful 
objection to your theory of attraction, that I doubt whether 
even your conjuror Newton, with his magic wand of attrac¬ 
tion, will be able to dispel it. 

Mrs. B. Well, my dear, pray what is this weighty ob¬ 
jection? 

Caroline. You say that bodies attract in proportion to 
the quantity of matter they contain; now we all know the 
sun to be much larger than the earth: why, therefore, does 
it not attract the earth; you will not, I suppose, pretend to 
say that we are falling towards the sun? 

Emily. However plausible your objection appears, Car¬ 
oline, I think you place too much reliance upon it: when 
any one has given such convincing proofs of sagacity and 
wisdom as Sir Isaac Newton, when we find that his opin¬ 
ions are universally received and adopted, is it to be ex¬ 
pected that any objection we can advance should overturn 
them ? 

Caroline. Yet I confess that I am not inclined to yield 
implicit faith even to opinions of the great Newton: for 
what purpose are we endowed with reason, if we are de- 


£5j) CAUSES OB' THE EARTH’S ANNUAL MOTION. 

med the privilege of making use of it, by judging for our¬ 
selves. 

Mrs. B. It is reason itself which teaches us, that when 
we, novices in science, start objections to theories establish¬ 
ed by men of knowledge and wisdom, we should be diffi¬ 
dent rather of our own than of their opinion. I am far from 
wishing to lay the least restraint on your questions; you can 
not be better convinced of the truth of a system, than by find¬ 
ing that it resists all your attacks, but I would advise you 
not to advance your objections with so much confidence, 
in order that the discovery of their fallacy may be attend¬ 
ed with less mortification. In answer to that you have just 
proposed, I can only say, that the earth really is attracted 
by the sun. 

Caroline f Take care at least that we are not consumed 
by him, Mrs. B. 

Mrs . B. We are in no danger; but our magician, New¬ 
ton, as you are pleased to call him, can not extricate him¬ 
self from this difficulty without the aid of some cabalistical 
figures, which I must draw for him. 

Let us suppose the earth, at its creation, to have been 
projected forwards into universal space: we know that if 
no obstacle impeded its course it would proceed in the same 
direction, and with a uniform velocity forever. In fig. 1. 
plate VI, A represents the earth, and S the sun. We shall 
suppose the earth to be arrived at the point in which it is 
represented in the figure, having a velocity which would 
carry it on to B in the space of one month; whilst the sun’s 
attraction would bring it to C in the same space of time. 
Observe that the two forces of projection and attraction do 
not act in opposition, but perpendicularly, or at a right an¬ 
gle to each other. Can you tell me now, how the earth 
will move? 

Emily. I recollect your teaching us that a body acted 
upon by two forces perpendicular to each other, would move 
in the diagonal of a parallelogram; if, therefore, I complete 
the parallelogram by drawing the lines C D, B D, the earth 
will move in the diagonal A D. 

Mrs. B. A ball struck by two forces acting perpendicu- 


I’J.ATK VI. 





























































CAUSES OP THE EARTH’S ANNUAL MOTION. 81 

larly to each other, it is true, moves in the diagonal of a 
parallelogram; but you must observe that the force of at¬ 
traction is continually acting upon our terrestrial ball, and 
producing an incessant deviation from its course in a right 
line, which converts it into that of a curve-line; every point 
of which may be considered as constituting the diagonal of 
an infinitely small parallelogram. 

Let us detain the earth a moment at the point D, and 
consider how it will be affected by the combined action of 
the two forces in its new situation. It still retains its ten¬ 
dency to fly off in a straight line; but a straight line would 
now carry it away to F, whilst the sun would attract it in 
the direction D S; how then will it proceed? 

—Emily. It will go on in a curve-line, in a direction be¬ 
tween that of the two forces. 

Mrs. B. In order to know exactly what course the earth 
will follow, draw another parallelogram similar to the first, 
in which the line D F describes the force of projection, and 
the line D S that of attraction; and you will find that the 
earth will proceed in the curve-line D G. 

Caroline. You must now allow me to draw a parallel¬ 
ogram, Mrs. B. Let me consider in what direction will 
the force of projection now impel the earth. 

Mrs. B. First draw a line from the earth to the sun 
representing the force of attraction; then describe the force 
of projection at a right angle to it. 

Caroline. The earth will then move in the curve G I, 
of the parallelogram G H I K. 

Mrs. B. You recollect that a body acted upon by two 
forces, moves through a diagonal in the same time that it 
would have moved through one of the sides of the parallel¬ 
ogram, were it acted upon by one force only. The earth 
has passed through the diagonals of these three parallelo¬ 
grams in the space of three months, and has performed one 
quarter of a circle; and on the same principle it will go on 
till it has completed the whole of the circle. It will then 
recommence a course, which it has pursued ever since it 
first issued from the hand of its Creator, and which there 
8 





§2 CAUSES OP THE EARTH’S ANNUAL MOTION. 

is every reason to suppose it will continue to follow, as long 
as it remains in existence. 

Emily. What a grand and beautiful effect resulting from 
so simple a cause! 

Caroline. It affords an example, on a magnificent scale, 
of the circular motion which you taught us in mechanics. 
The attraction of the sun is the centripetal force, which 
confines the earth to a centre, and the impulse of projection 
the centrifugal force, which impels the earth to quit the sun 
and fly off in a tangent. 

Mrs. B. Exactly so. A simple mode of illustrating the 
effect of these combined forces on the earth, is to cut a slip 
of card in the form of a right angle, (fig. 2. plate VI.) to 
describe a small circle at the angular point representing the 
earth, and to fasten the extremity of one of the legs of the 
angle to a fixed point, which we shall consider as the sun. 
Thus situated, the angle will represent both the centrifugal 
and centripetal forces; and if you draw it round the fixed 
point, you will see how the direction of the centrifugal force 
varies, constantly forming a tangent to the circle in which 
ihe earth moves, as it is constantly at a right .angle with the 
centripetal force. 

Emily. The earth then, gravitates towards the sun with¬ 
out the slightest danger either of approaching nearer or re¬ 
ceding further from it. How admirably this is contrived! 
If the two forces which produce this circular motion had 
net been so accurately adjusted, one would ultimately have 
prevailed over the other, and we should either have ap¬ 
proached so near the sun as to have been burnt, or have 
receded so far from it as to have been frozen. 

Mrs. B. What will you say, my dear, when I tell you, 
that these two forces are not, in fact, so proportioned as to 
produce circular motion in the earth? 

Caroline. You must explain to us, at least, in what man¬ 
ner we avoid the threatened destruction. 

Mrs. B. Let us suppose that when the earth is at A, 
(fig. 3.) its projectile force should not have given it a ve¬ 
locity sufficient to counterbalance that of gravity, so as to 
enable these powers conjointly to carry it round the sun in 


CAUSES OF THE EARTH’S ANNUAL MOTION. 83 

a circle; the earth, instead of describing the line A C, as 
in the former figure, will approach nearer the sun in the 
line A B. 

Caroline. Under these circumstances, I see not what is 
to prevent our approaching nearer and hearer the sun till 
we fall into it: for its attraction increases as we advance 
towards it, and produces an accelerated velocity in the 
earth, which increases the danger. 

Mrs. B. And there is yet another danger, of which you 
are not aware. Observe, that as the earth approaches the 
sun, the direction of its projectile force is no longer perpen¬ 
dicular to that of attraction, but inclines more nearly to it. 
When the earth reaches that part of its orbit at B, the force 
of projection would carry it to D, which brings it nearer 
the sun instead of bearing it away from it. 

Emily. If, then, we are driven by one power and drawn 
by the other to this centre of destruction, how is it possible 
for us to escape? 

Mrs. B. A little patience, and you will find that we are 
not without resource. The earth continues approaching the 
sun with a uniformly increasing accelerated motion, till it 
reaches the point E; in what direction will the projectile 
force now impel it? 

Emily. In the direction EF. Here then the two forces 
act perpendicularly to each other, and the earth is situated 
just as it was in the preceding figure; therefore, from this 
point, it should revolve round the sun in a circle. 

Mrs. B. No, all the circumstances do not agree. In mo¬ 
tion round a centre, you recollect that the centrifugal force 
increases with the velocity of the body, or in other words, 
the quicker it moves the stronger is its tendency to fly off 
in a right line. When the earth, therefore, arrives at E, 
its accelerated motion will have so far increased its veloci¬ 
ty, and consequently its centrifugal force, that the latter 
will prevail over the force of attraction, and drag the earth 
away from the sun till it reaches G. 

Caroline. It is thus then that we escape from the dan¬ 
gerous vicinity of the sun; and in proportion as we recede 
from it, the force of its attraction, and, consequently, the 
velocity of the earth’s motion, are diminished. 


84 CAUSES OF THE EARTH’S ANNUAL MOTION. 

Mrs. B. Yes. From G the direction of projection is 
towards H, that of attraction towards S, and the earth pro¬ 
ceeds between them with a uniformly retarded motion, till 
it has completed its revolution. Thus you see that the 
earth travels round the sun, not in a circle but an ellipsis, 
of which the sun occupies one of the foci; and that in its 
course the earth alternately approaches and recedes from 
it, without any danger of being either swallowed up, or be¬ 
ing entirely carried away from it. 

Caroline. And I observe, that what I apprehended to be 
a dangerous irregularity, is the means by which the most 
perfect order and harmony are produced. 

Emily. The earth travels then at a very unequal rate, 
Us velocity being accelerated as it approaches the sun, and 
retarded as it recedes from it. 

Mrs. B. It is mathematically demonstrable, that, in 
moving round a point towards which it is attracted, a body 
passes over equal areas in equal times. The whole of the 
space contained within the earth’s orbit, is in fig. 4, divided 
into a number of areas or spaces, 1,2, 3, 4, &c. all of which 
are of equal dimensions, though of very different forms; 
some of them, you see, are long and narrow, others broad 
and short: but they each of them contain an equal quantity 
of space. An imaginary line drawn from the centre of the 
earth to that of the sun, and keeping pace with the earth in 
its revolution, passes over equal areas in equal times; that 
is to say, if it is a month going from A to B, it will be a 
month going from B to C, and another from C to E, and 
so on. 

Caroline. What long journeys the earth has to perform 
in the course of a month, in one part of her orbit, and how 
short they are in the other part! 

Mrs. B. The inequality is not so considerable as ap¬ 
pears in this figure; for the earth’s orbit is not so eccentric 
as it is there described; and in reality, differs but little from 
a circle: that part of the earth’s orbit nearest the sun is call¬ 
ed its perihelion, that part most distant from the sun its 
aphelion; and the earth is above three millions of miles 
nearer the sun at its perihelion than at its aphelion. 


CAUSES OF THE EARTH’S ANNUAL MOTION. 85 

Emily. I think I can trace a consequence from these dif¬ 
ferent situations of the earth; is it not the cause of summer 
and winter? 

Mrs. B. On the contrary, during the height of summer, 
the earth is in that part of its orbit which is most distant 
from the sun, and it is during the severity of winter, that it 
approaches nearest to it. 

Emily. That is very extraordinary; and how then do 
you account for the heat being greatest, when we are most 
distant from the sun? 

Mrs. B. The difference of the earth’s distance from the 
sun in summer and winter, when compared with its total 
distance from the sun, is but inconsiderable. The earth, it 
is true, is above three millions of miles nearer the sun in 
winter than in summer; but that distance, however great it 
at first appears, sinks into insignificance in comparison with 
95 millions of miles, which is our mean distance from the 
sun. The change of temperature, arising from this difference, 
would scarcely be sensible, were it not completely over¬ 
powered by other causes which produce the variations of 
the seasons; but these I shall defer explaining, till we have 
made some further observations on the heavenly bodies. 

Caroline. And should not the sun appear smaller in 
summer, when it is so much further from us? 

Mrs. B. It actually does, when accurately measured; 
but the apparent difference in size, is, I believe, not percep¬ 
tible to the naked eye. 

Emily. Then, since the earth moves with greatest velo¬ 
city in that part of its orbit nearest the sun, it must have 
completed its journey through one half of its orbit in a short¬ 
er time than the other half? 

Mrs. B. Yes, it is about seven days longer performing 
the summer-half of its orbit than the winter-half. 

The revolution of all the planets round the sun is the re¬ 
sult of the same causes, and is performed in the same man¬ 
ner as that of the earth. 

Caroline. Pray what are the planets? 

Mrs . B. They are those celestial bodies, which revolve 
like our earth about the sun; they are supposed to resemble 
8 * 


86 CAUSES OF THE EARTH’S ANNUAL MOTION. 

the earth also in many other respects; and we are led by 
analogy to suppose them to be inhabited worlds. 

Caroline. I have heard so, but do you not think such 
an opinion too great a stretch of the imagination? 

Mrs. B. Some of the planets are proved to be larger 
than the earth; it is only their immense distance from us, 
which renders their apparent dimensions so small. Now, 
if we consider them as enormous globes, instead of small 
twinkling spots, we shall be led to suppose that the Almighty 
would not have created them merely for the purpose of 
giving us a little light in the night, as it was formerly ima¬ 
gined, and we should find it more consistent with our ideas 
of the Divine wisdom and beneficence to suppose that these 
celestial bodies, should be created for the habitation of be¬ 
ings, who are, like us, blessed by his providence. Both in 
a moral as well as a physical point of view, it appears to me 
more rational to consider the planets as worlds revolving 
round the sun; and the fixed stars as other suns, each of 
them attended by their respective system of planets, to which 
they impart their influence. We have brought our tele¬ 
scopes to such a degree of perfection, that from the appear¬ 
ances which the moon exhibits when seen through them, 
we have very good reason to conclude that it is a habitable 
globe, for though it is true that t we can not discern its towns 
and people, we can plainly perceive its mountains and val¬ 
leys; and some astronomers have gone so far as to imagine 
they discovered volcanos. 

Emily. If the fixed stars are suns, with planets revolv¬ 
ing round them, why should we not see those planets as well 
as their suns? 

Mrs. B. In the first place, we conclude that the planets 
of other systems (like those of our own) are much smaller 
than the suns which give them light; therefore at so great 
a distance as to make the suns appear like fixed stars, the 
planets would be quite invisible. Secondly, the light of the 
planets being only reflected light, is much more feeble than 
that of the fixed stars. There is exactly the same differ¬ 
ence as between the light of the sun and that of the moon; 
the first being a fixed star, the second a planet. 


CAUSES OP THE EARTH’S ANNUAL MOTION. 87 

Emily. But if the planets are worlds like our earth, they 
are dark bodies; and instead of shining by night, we should 
see them only by day-light. And why do we not see the 
fixed stars also by day-light? 

Mrs. B. Both for the same reason; their light is so 
faint, compared to that of our sun reflected by the atmos¬ 
phere, that it is entirely effaced by it: the light emitted by 
the fixed stars may probably be as strong as that of our sun, 
at an equal distance; but being so much more remote, it is 
diffused over a greater space, and is consequently propor¬ 
tionally weakened. 

Caroline. True; 1 can see much better by the light of 
a candle that is near me, than by that of one at a great dis¬ 
tance. But I do not understand what makes the planets 
shine? 

Mrs. B. What is that which makes the steel buttons on 
your brother’s coat shine? 

Caroline. The sun. But if it was the sun which made 
the planets shine, we should see them in the day-time, when 
the sun shone upon them; or if the faintness of their light 
prevented our seeing them in the day, we should not see 
them at all, for the sun can not shine upon them in the night. 

Mrs. B. There you are in error. But in order to ex¬ 
plain this to you, I must first make you acquainted with the 
various motions of the planets. 

You know, that according to the laws of attraction, the 
planets belonging to our system all gravitate towards the 
sun; and that this force, combined with that of projection, 
will occasion their revolution round the sun, in orbits more 
or less elliptical, according to the proportion which these 
two forces bear to each other. 

But the planets have also another motion: they revolve 
upon their axis. The axis of a planet is an imaginary line 
which passes through its centre, and on which it turns; and 
it is this motion which produces day and night. With that 
side of the planet facing the sun it is day; and with the 
opposite side, which remains in darkness, it is night. Our 
earth, which we consider as a planet, is 24 hours in perform¬ 
ing one revolution on its axis; in that period of time, there- 


8$ CAUSES OF THE EARTH’S ANNUAL MOTION. 

fore, we have a day and a night; hence this revolution is 
called the earth’s diurnal or daily motion; and it is this re¬ 
volution of the earth from west to cast which produces an 
apparent motion of the sun, moon and stars, in a contrary 
direction. 

Let us now suppose ourselves to be beings independent 
of any planet, travelling in the skies, and looking upon the 
earth in the same point of view as upon the other planets. 

Caroline. It is not flattering to us, its inhabitants, to see 
it make so insignificant an appearance. 

Mrs. B. To those who are accustomed to contemplate 
it in this light, it never appears more glorious. We are 
taught by science to distrust appearances; and instead of 
considering the planets as little stars, we look upon them 
either as brilliant suns or habitable worlds, and we consid¬ 
er the whole together as forming one vast and magnificent 
system, worthy of the Divine hand by which it was created. 

Emily. I can scarcely conceive the idea of this immen¬ 
sity of creation; it seems too sublime for our imagination;— 
and to think that the goodness of Providence extends over 
millions of worlds throughout a boundless universe—Ah! 
Mrs. B., it is we only who become trifling and insignificant 
beings in so magnificent a creation! 

Mrs. B. This idea should teach us humility, but with¬ 
out producing despondency. The same Almighty hand 
which guides these countless worlds in their undeviating 
course, conducts with equal perfection the blood as it cir¬ 
culates through the veins of a fly, and opens the eye of the 
insect to behold His wonders. Notwithstanding this im¬ 
mense scale of creation, therefore, we need not fear to be 
disregarded or forgotten. 

But to return to our station in the skies. We were, if 
you recollect, viewing the earth at a great distance, in ap¬ 
pearance a little star, one side illumined by the sun, the 
other in obscurity. But would you believe it, Caroline, 
many of the inhabitants of this little star imagine that when 
that part which they inhabit is turned from the sun, dark¬ 
ness prevails throughout the universe, merely because it is 
night with them; whilst, in reality, the suu never ceases to 


CAUSES OF THE EARTH’S ANNUAL MOTION. 89 

shine upon every planet. When, therefore, these little ig¬ 
norant beings look around them during their night, and be¬ 
hold all the stars shining, they can not imagine why the 
planets, which are dark bodies, should shine; concluding, 
that since the sun does not illumine themselves, the whole 
universe must be in darkness. 

Caroline . I confess that I was one of these ignorant 
people; but I am now very sensible of the absurdity ofsuch 
an idea. To the inhabitants of the others planets, then, we 
must appear as a little star? 

Mrs. B. Yes, to those which revolve round our sun; for 
since those which may belong to other systems, (and whose 
existence is only hypothetical) are invisible to us, it is pro¬ 
bable that we also are invisible to them. 

Emily. But they may see our sun as we do theirs, in 
appearance a fixed star? 

Mrs. B. No doubt; if the beings who inhabit those pla¬ 
nets are endowed with senses similar to ours. By the same 
rule we must appear as a moon to the inhabitants of our 
moon; but on a larger scale, as the surface of the earth is 
about thirteen times as large as that of the moon. 

Emily. The moon, Mrs. B., appears to move in a dif¬ 
ferent direction, and in a different manner from the stars? 

Mrs. B. I shall defer the explanation of the motion of 
the moon till our next interview, as it would prolong our 
present lesson too much. 


CONVERSATION VII. 


OP THE PLANETS. 

OF THE SATELLITES OH MOONS.-GRAVITY DIMINISHES AS THE SQ.UARE OF 

THE DISTANCE.-OF THE SOLAR SYSTEM.-OF COMETS.-CONSTELLA¬ 
TIONS, SIGNS OF THE ZODIAC.-OF COPERNICUS, NEWTON, &C. 


MRS. B. 

The planets are distinguished into primary and seconda¬ 
ry. Those which revolve immediately about the sun are 
called primary. Many of these are attended in their course 
by smaller planets, which revolve round them: these are 
called secondary planets, satellites, or moons. Such is 
our moon which accompanies the earth, and is carried with 
it round the sun. 

Emily . How then can you reconcile the motion of the 
secondary planets to the laws of gravitation; for the sun is 
much larger than any of the primary planets; and is not the 
power of gravity proportional to the quantity of matter? 

Caroline. Perhaps the sun, though much larger, may 
he less dense than the planets. Fire you know, is very 
light, and it may contain but little matter, though of great 
magnitude. 

Mrs. B. We do not know of what kind of matter the 
sun is made; but we may be certain, that since it is the gen¬ 
eral centre of attraction of our system of planets, it must be 
the body which contains the greatest quantity of matter in 
that system. 

You must recollect, that the force of attraction is not on¬ 
ly proportional to the quantity of matter, but to the degree 


ON THE PLANETS. 


91 


of proximity of the attractive body: this power is weakened 
by being diffused, and diminishes as the scjuares of the dis¬ 
tances increase. The square is the product of a number 
multiplied by itself; so that a planet situate at twice the 
distance at which we are from the sun would gravitate four 
times less than we do; for the product of two multiplied by 
itself is four. 

Caroline, Then the more distant planets move slower 
in their orbits; for their projectile force must be propor¬ 
tioned to that of attraction? But I do not see how this 
accounts for the motion of the secondary round the primary 
planets, in preference to the sun? 

Emily . Is it not because the vicinity of the primary 
planets renders their attraction stronger than that of the 
sun? 

J\Irs. B. Exactly so. But since the attraction between 
bodies is mutual, the primary planets are also attracted by 
the satellites which revolve round them. The moon attracts 
the earth, as well as the earth the moon; but as the latter 
is the smaller body, her attraction is proportionally less; 
therefore neither the earth revolves round the moon, nor 
the moon round the earth; but they both revolve round a 
point, which is their common centre of gravity, and which 
is as much nearer the earth than the moon, as the gravity 
of the former exceeds that of the latter. 

Emily. Yes, I recollect your saying, that if two bodies 
were fastened together by a wire or bar, their common cen¬ 
tre of gravity would be in the middle of the bar, provided 
the bodies were of equal weight; and if they differed in 
weight, it would be nearer the larger body. If then the 
earth and moon had no projectile force which prevented 
their mutual attraction from bringing them together, they 
would meet at their common centre of gravity. 

Caroline. The earth then has a great variety of motion, 
it revolves round the sun, upon its axis, and round the point 
towards which the moon attracts it. 

Mrs. B t Just so; and this is the case with every planet 
which is attended by satellites. The complicated effect of 


92 


ON THE PLANETS. 


this variety of motions, produces certain irregularities which, 
however, it is not necessary to notice at present. 

The planets act on the sun in the same manner as they 
are themselves acted on by their satellites; for attraction, 
you must remember, is always mutual; but the gravity of 
the planets (even when taken collectively) is so trifling 
compared with that of the sun, that they do not cause the 
latter to move so much as one half of his diameter. The 
planets do not, therefore, revolve round the centre of the 
sun, but round a point at a small distance from its centre, 
about which the sun also revolves. 

Emily. I thought the sun had no motion? 

Mrs. B. You were mistaken; for besides that which I 
have just mentioned, which is indeed very inconsiderable, 
he revolves on his axis; this motion is ascertained by ob¬ 
serving certain spots which disappear, and reappear regu¬ 
larly at stated times. 

Caroline . A planet has frequently been pointed out to 
me in the heavens; but I could not perceive that its mo¬ 
tion differed from that of the fixed stars, which only appear 
to move. 

Mrs. B. The great distance of the planets renders their 
motion apparently so slow, that the eye is not sensible of 
their progress in their orbit, unless we watch them for some 
considerable length of time: in different seasons they ap¬ 
pear in different parts of the heavens. The most accurate 
idea I can give you of the situation and motion of the plan¬ 
ets, will be by the examination of this diagram, (plate VII. 
fig. 1) representing the solar system, in which you will find 
every planet with its orbit delineated. 

Emily. But the orbits here are all circular, and you 
said that they were elliptical. The planets appear too, to 
be moving round the centre of the sun; whilst you told 
us that they moved round a point at a little distance from 
thence. 

Mrs. B. The orbits of the planets are so nearly cir¬ 
cular, and the common centre of gravity of the solar sys¬ 
tem so near the centre of the sun, that these deviations 
are scarcely worth observing. The dimensions of the 


Pj*atetu. 


Fin. 1 . 








Ilerany J/,rs 

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ON THE PLANETS# 93 

planets, in their true proportions, you will find delineated 
in fig. 2. 

Mercury is the planet nearest the sun; his orbit is conse¬ 
quently contained within ours; but his vicinity to the sun, 
occasions his being nearly lost in the brilliancy of his rays; 
and when we see the sun, he is so dazzling, that very ac¬ 
curate observations can not be made upon Mercury. He 
performs his revolution round the sun in about 8? days, 
which is consequently the length of his year. The time 
of his rotation on his axis is not known; his distance from 
the sun is computed to be 37 millions of miles, and his di¬ 
ameter 3180 miles. The heat of this planet is so great, 
that water can not exist there but in a state of vapour, and 
metals would be liquified. 

Caroline. Oh, what a dreadful climate! 

Mrs. B. Though we could not live there, it may be per¬ 
fectly adapted to other things destined to inhabit it. 

Venus, the next in the order of planets, is 68 millions of 
miles from the sun: she revolves about her axis in 23 hours 
and 21 minutes, and goes round the sun in 244 days 17 
hours. The orbit of Venus is also within ours; during one 
half of her course in it, we see her before sun-rise, and she 
is called the morning star; in the other part of her orbit she 
rises later than the sun. 

Caroline. In that case we can not see her, for she must 
rise in the day time? 

Mrs. B. True; but when she rises later than the sun, 
she also sets later; so that we perceive her approaching 
the horizon after sun-set: she is then called Hesperus, or 
the evening star. Do you recollect those beautiful lines of 
Milton: 

Now came still evening on, and twilight gray 
Had in her sober livery all things clad; 

Silence accompanied; for beast and bird, 

They to their grassy couch, these to their nests 
Were slunk, all but the wakeful nightingale; 

She all night long her amorous descant sung; 

Silence was pleased; now glowed the firmament 

With living saphirs; Hesperus that led 

The starry host, rode brightest, till the moon 


94 


ON THE PLANETS. 


Rising in clouded majesty, at length 
Apparent queen unveil’d her peerless light. 

And o’er the dark her silver mantle threw. 

The Planet next to Venus is the Earth, of which we shall 
soon speak at full length. At present I shall only observe 
that we are 95 millions of miles distant from the sun, that 
we perform our annual revolution in 365 days 5 hours and 
49 minutes; and are attended in our course by a single 
moon. 

Next follows Mars. He can never Come between us and 
the sun, like Mercury and Venus; his motion is, however, 
very perceptible, as he may be traced to different situations 
in the heavens; his distance from the sun is 144 millions of 
miles; he turns round his axis in 24 hours and 39 miuutes; 
and he performs his annual revolution, in about 687 of our 
days: his diameter is 4120 miles. Then follow four very 
small planets, Juno, Ceres, Pallas and Vesta, which have 
been recently discovered, but whose dimensions and dis¬ 
tances from the sun have not been very accurately ascer¬ 
tained. 

Jupiter is next in order: this is the largest of all the pla¬ 
nets. He is about 490 millions of miles from the sun, and 
completes his annual period in nearly 12 of our years. He 
turns round his axis in about ten hours. He is above 1200 
times as big as our earth; his diameter is 86,000 miles. 
The respective proportions of the planets can not, there¬ 
fore, you see, be conveniently delineated in a diagram. He 
is attended by four moons. 

The next planet is Saturn, whose distance from the sun 
is about 900 millions of miles; his diurnal rotation is per¬ 
formed in 10 hours and a quarter: his annual revolution is 
nearly 30 of our years. His diameter is 79,000 miles. 
This planet is surrounded by a luminous ring, the nature of 
which, astronomers are much at a loss to conjecture: he 
has seven moons. Lastly, we observe the Georgium Sidus, 
discovered by Dr, Herschel, and which is attended by six 
moons. 

Caroline. How charming it must be in the distant pla- 


ON THE PLANETS. 95 

nets, to see several moons shining at the same time; I think 
I should like to be an inhabitant of Jupiter or Saturn. 

Mrs. B. Not long I believe. Consider what extreme 
cold must prevail in a planet, situated as Saturn is, at near¬ 
ly ten times the distance at which we are from the sun. 
Then his numerous moons are far from making so splendid 
an appearance as ours; for they can reflect only the light 
which they receive from the sun; and both light and heat 
decrease in the same ratio or proportion to the distances as 
gravity. Can you tell me now how much more light we 
enjoy than Saturn? 

Caroline. The square of ten is a hundred; therefore, Sa¬ 
turn has a hundred times less—or to answer your question 
exactly, we have a hundred times more light and heat than 
Saturn—this certainly does not increase my wish to become 
one of the poor wretches who inhabit that planet. 

Mrs. B . May not the inhabitants of Mercury, with equal 
plausibility, pity us for the insupportable coldness of our 
situation; and those of Jupiter and Saturn for our intolera¬ 
ble heat? The Almighty Power which created these planets, 
and placed them in their several orbits, has no doubt peo¬ 
pled them with beings whose bodies are adapted to the va¬ 
rious temperatures and elements in which they are situated. 
If we judge from the analogy of our own earth; or from that 
of the great and universal beneficence of Providence, we 
must conclude this to be the case. 

Caroline. Are not comets also supposed to be planets? 

Mrs. B. Yes, they are; for by the reappearance of some 
of them, at stated times, they are known to revolve round 
the sun, but in orbits so extremely eccentric, that they dis¬ 
appear for a great number of years. If they are inhabited, 
it must be by a species of beings very different, not only 
from the inhabitants of this, but from those of any of the 
other planets, as they must experience the greatest vicissi¬ 
tude's of heat and cold; one part of their orbit being so near 
the sun, that their heat, when there, is computed to be 
greater than that of red-hot iron; in this part of its orbit, 
the comet emits a luminous vapour, called the tail, which 
it gradually loses as it recedes from the sun; and the comet 


36 


ON THE PLANETS. 


itself totally disappears from our sight, in the more distant 
parts of its orbit, which extends considerably beyond that 
of the furthest planet. 

The number of comets belonging to our system can not 
be ascertained, as some of them are whole centuries before 
they make their reappearance. The number that are known 
by their regular reappearance is only three. 

Emily. Pray, Mrs. B., what are the constellations? 

Mrs. B. They are the fixed stars, which the ancients, 
in order to recognise them, formed into groups, and gave 
the names of the figures, which you find delineated on the 
celestial globe. In order to show their proper situations in 
the heavens, they should be painted on the internal surface 
of a hollow sphere, from the centre of which you should 
view them; you would then behold them as they appear to 
be situated in the heavens. The twelve constellations, call¬ 
ed the signs of the zodiac, are those which are so situated, 
that the earth, in its annual revolution, passes directly be¬ 
tween them and the sun. Their names are Aries, Taurus, 
Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, 
Capricornus, Aquarius, Pisces; the whole occupying a com¬ 
plete circle, or broad belt, in the heavens, called the zodiac, 
{plate VIII. fig. 1.) Hence, a right line drawn from the 
earth, and passing through the sun, would reach one of these 
constellations, and the sun is said to be in that constella¬ 
tion at which the line terminates: thus, when the earth is 
at A, the sun would appear to be in the constellation or 
sign Aries; when the earth is at B, the sun would appear 
in Cancer; when the earth was at C, the sun would be in 
Libra; and when the earth was at D, the sun would be in 
Capricorn. This circle, in which the sun thus appears to 
move, and which passes through the middle of the zodiac, 
is called the ecliptic. 

Caroline. But many of the stars in these constellations 
appear beyond the zodiac. 

Mrs. B. We have no means of ascertaining the distance 
of the fixed stars. When, therefore, they are said to be in 
the zodiac, it is merely implied that they are situated in 


i > LATK Vm. 























































- > 






ON THE PLANETS. 97 

that direction, and that they shine upon us through that por¬ 
tion of the heavens, which we call the zodiac. 

Emily . But are not those large bright stars, which are 
called stars of the first magnitude, nearer to us, than those 
small ones which we can scarcely discern? 

Mrs. B. It maybe so; or the difference of size and bril¬ 
liancy of the stars may proceed from their difference of di¬ 
mensions; this is a point which astronomers are not ena¬ 
bled to determine. Considering them as suns, I see no 
reason why different suns should not vary in dimensions, as 
well as the planets belonging to them. 

Emily . What a wonderful and beautiful system this is, 
and how astonishing to think that every fixed star may pro¬ 
bably be attended by a similar train of planets! 

Caroline. You will accuse me of being very incredu¬ 
lous, but I can not help still entertaining some doubts, and 
fearing that there is more beauty than truth in this system. 
It certainly may be so; but there does not appear to me to 
be sufficient evidence to prove it. It seems so plain and 
obvious that the earth is motionless, and that the sun and 
stars revolve round it;—your solar system, you must allow, 
is directly in opposition to the evidence of our senses. 

Mrs. B. Our senses so often mislead us, that we should 
not place implicit reliance upon them. 

Caroline. On what then can we rely, for do we not re¬ 
ceive all our ideas through the medium of our senses? 

Mrs. B. It is true that they are our primary source of 
knowledge; but the mind has the power of reflecting, judg¬ 
ing, and deciding upon the ideas received by the organs of 
sense. This faculty, which we call reason, has frequently 
proved to us, that our senses are liable to err. If you have 
ever sailed on the water, with a very steady breeze, you 
must have seen the houses, trees, and every object move, 
while you were sailing. 

Caroline. I remember thinking so, when I was very 
young; but I now know that their motion is only apparent. 
It is true that my reason, in this case, corrects the error of 
my sight. 

Mrs. B. It teaches you, that the apparent motion of the 
9 * 


9S 


ON THE PLANETS. 


objects on shore, proceeds from, your being yourself moving, 
and that you are not sensible of your own motion, because 
you meet with no resistance. It is only when some obsta¬ 
cle impedes our motion, that we are conscious of moving; 
and if you were to close your eyes when you were sailing 
on calm water, with a steady wind, you would not perceive 
that you moved, for you could not feel it, and you could see 
it only by observing the change of place of the objects on 
shore. So it is with the motion of the earth: every thing 
on its surface, and the air that surrounds it, accompanies it 
in its revolution; it meets with no resistance: therefore, like 
the crew of a vessel sailing with a fair wind, in a calm sea, 
we are insensible of our motion. 

Caroline, But the principal reason why the crew of a 
vessel in a calm sea do not perceive their motion, is, be¬ 
cause they move exceedingly slow, while the earth, you say, 
revolves with great velocity. 

Mrs. B. It is not because they move slowly, but be¬ 
cause they move steadily, and meet with no irregular re¬ 
sistances, that the crew of a vessel do not perceive their 
motion; for they would be equally insensible to it, with the 
strongest wind, provided it were steady, that they sailed 
with it, and that it did not agitate the water; but this last 
condition, you know, is not possible, for the wind will al¬ 
ways produce waves which offer more or less resistance to 
the vessel, and then the motion becomes sensible, because 
it is unequal. 

Caroline. But, granting this, the crew of a vessel have 
a proof of their motion, though insensible, which the inha¬ 
bitants of the earth can not have,—the apparent motion of 
the objects on shore. 

Mrs. B. Have we not a similar proof of the earth’s 
motion in the apparent motion of the sun and stars? Ima¬ 
gine the earth to be sailing round its axis, and successively 
passing by every star, which, like the objects on land, we 
suppose to be moving instead of ourselves. I have heard it 
observed by an aerial traveller in a balloon, that the earth 
appears to sink beneath the balloon, instead of the balloon 
rising above the earth. 


ON THE PLANETS. 


99 


It is a law which we discover throughout nature, and 
worthy of its great Author, that ail its purposes are accom¬ 
plished by the most simple means; and what reason have 
we to suppose this law infringed, in order that we may re¬ 
main at rest, while the sun and stars move round us; their 
regular motions, which are explained by the laws of attrac¬ 
tion on the first supposition, would be unintelligible on the 
last, and the order and harmony of the universe be destroy¬ 
ed. Think what an immense circuit the sun and stars 
would make daily, were their apparent motions real. We 
know many of them to be bodies more considerable than 
our earth; for our eyes vainly endeavour to persuade us, 
that they are little brilliants sparkling in the heavens, while 
science teaches us that they are immense spheres, whose 
apparent dimensions are diminished by distance. Why then 
should these enormous globes daily traverse such a prodi¬ 
gious space, merely to prevent the necessity of our earth’s 
revolving on its axis? 

Caroline. I think I must now be convinced. But you 
will, I hope, allow me a little time to familiarise myself to 
an idea so different from that which I have been accustom¬ 
ed to entertain. And pray, at what rate do we move? 

Mrs. B. The motion produced by the revolution of the 
earth on its axis, is about eleven miles a minute, to an in¬ 
habitant of London. 

Emily. But does not every part of the earth move w r ith 
the same velocity? 

Mrs. B. A moment’s reflection would convince you of 
the contrary: a person at the equator must move quicker 
than one situated near the poles, since they both perform a 
revolution in 24 hours. 

Emily. True, the equator is farthest from the axis of 
motion. But in the earth’s revolution round the sun, every 
part must move with equal velocity? 

Mrs. B. Yes, about a thousand miles a minute. 

Caroline. How astonishing!—and that it should be pos¬ 
sible for us to be insensible of such a rapid motion. You 
would not tell me this sooner, Mrs. B. for fear of increas¬ 
ing my incredulity. 

Before the time of Newton, was not the earth supposed 


100 


ON THE PLANETS. 


to be in the centre of the system, and the sun, moon, and 
stars to revolve round it? 

Mrs, B. This was the system of Ptolemy in ancient 
times; but as long ago as the beginning of the sixteenth 
century it w ? as discarded, and the solar system, such as I 
have shown you, was established by the celebrated astro¬ 
nomer Copernicus, and is hence called the Copernican 
system. But the theory of gravitation, the source from 
which this beautiful and harmonious arrangement flows, 
we owe to the powerful genius of Newton, who lived at a 
much later period. 

Emily. It appears, indeed, far less difficult to trace by 
observation the motion of ^he planets, than to divine by 
what power they are impelled and guided. I wonder how 
the idea of gravitation could first have occurred to sir Isaac 
Newton? 

Mrs. B. It is said to have been occasioned by a cir¬ 
cumstance from which one should little have expected so 
grand a theory to have arisen. 

During the prevalence of the plague in the year 1665, 
Newton retired inte the country to avoid the contagion: 
when sitting one day in his orchard, he observed an apple 
fall from a tree, and was led to consider what could be the 
cause which brought it to the ground. 

Caroline. If I dared to confess it, Mrs. B., I should say 
that such an inquiry indicated rather a deficiency than a 
superiority of intellect. I do not understand how any one 
can wonder at what is so natural and so common. 

Mrs. B. It is the mark of superior genius to find matter 
for wonder, observation, and research, in circumstances 
which, to the ordinary mind, appear trivial, because they 
are common, and with which they are satisfied, because 
they are natural, without reflecting that nature is our grand 
field of observation, tjiat within it is contained our w hole 
store of knowledge; in a word, that to study the works of 
nature, is to learn to appreciate and admire the wisdom of 
God. Thus, it was the simple circumstance of the fall of 
an apple, which led to the discovery of the laws upon which 
the Copernican system is founded; and whatever credit this 


ON THE PLANETS. 101 

system had obtained before, it now rests upon a basis from 
which it can not be shaken. 

Emily. This was a most fortunate apple, and more wor¬ 
thy to be commemorated than all those that have been sung 
by the poets. The apple of discord for which the goddesses 
contended; the golden apples by which Atalanta won the 
race; nay, even the apple which William Tell shot from 
* the head of his son, can not be compared to this! 


CONVERSATION Till. 


* 


ON THE EARTH. 


OF THE TERRESTRIAL GLOBE.—OF THE FIGURE OF THE EARTH.-—OF THE 

FENRULUM.-OF THE VARIATION OF THE SEASONS, AND OF THE LENGTH 

OF DATS AND NIGHTS.-OF TnE CAUSES OF THE HEAT OF SUMMER.—OF 

SOLAR, S2DERIAL, AND E<iUAL OR MEAN TIME. 

MRS. B. 

As the earth is the planet in which we are the most 
particularly interested, it is my intention this morning, to 
explain to you the effects resulting from its annual and di¬ 
urnal motions; but for this purpose it will be necessary to 
make you acquainted with the terrestrial globe: you have 
not either of you, I conclude, learnt the use of the globes? 

Caroline. No; I once indeed learnt by heart the names 
of the lines marked on the globe, but as 1 was informed 
they were only imaginary divisions, they did not appear to 
me worthy of much attention, and were soon forgotten. 

J\lrs. B. You suppose, then, that astronomers had been 
at the trouble of inventing a number of lines to little pur¬ 
pose. It will be impossible for me to explain to you the 
particular effects of the earth’s motion, without your hav¬ 
ing acquired a knowledge of these lines: in plate VIII. 
fig. 2 . you will find them all delineated: and you must 
learn them perfectly if you wish to make any proficiency 
in astronomy. 

Caroline. I was taught them at so early an age that 
I could not understand their meaning; and I have often 


ON THE EARTH. 103 

heard you say that the only use of words was to convey 
ideas. 

Mrs. B. The names of these lines would have conveyed 
ideas of the figures they were designed to express, though 
the use of these figures might at that time have been too 
difficult for you to understand. Childhood is the season 
when impressions on the memory are most strongly and 
most easily made: it is the period at which a large stock 
of ideas should be treasured up, the application of which 
we may learn when the understanding is more developed. 
It is, I think, a very mistaken notion that children should 
be taught such things only as they can perfectly understand. 
Had you been early made acquainted with the terms which 
relate to figure and motion, how much it would have faci¬ 
litated your progress in natural philosophy. I have been 
obliged to confine myself to the most common and familiar 
expressions, in explaining the laws of nature, though I am 
convinced that appropriate and scientific terms would have 
conveyed more precise and accurate ideas; but I was afraid 
of not being understood. 

Emily. You may depend upon our learning the names 
of these lines thoroughly, Mrs. B.; but before we commit 
them to memory, will you have the goodness to explain 
them to us? 

Mrs. B. Most willingly. This globe, or sphere, repre¬ 
sents the earth; the line which passes through its centre, 
and on which it turns, is called its axis, and the two extre¬ 
mities of the axis A and B, are the poles, distinguished by 
the names of the north and the south pole. The circle 
C D, which divides the globe into two equal parts between 
the poles, is called the equator, or equinoctial line; that 
part of the globe to the north of the equator is the northern 
hemisphere; that part to the south of the equator, the south¬ 
ern hemisphere. The small circle E F, which surrounds 
the north pole, is called the arctic circle, that G II, which 
surrounds the south pole, antarctic circle. There are two 
immediate circles between the polar circles and the equa¬ 
tor; that to the north, I K, called the tropic of Cancer; that 
to the south, L M, called the tropic of Capricorn. Lastly, 


104 


ON THE EARTH. 


this circle, L K, which divides the globe into two equal 
parts, crossing the equator and extending northward as far 
as the tropic of Cancer, and southward as far as the tropic 
of Capricorn, is called the ecliptic. The delineation of 
the ecliptic on the terrestrial globe is not without danger of 
conveying false ideas; for the ecliptic (as I have before 
said) is an imaginary circle in the heavens passing through 
the middle of the zodiac, and situated in the plane of the 
earth’s orbit. 

Caroline. I do not understand the meaning of the plane 
of the earth’s orbit. 

Mrs. B. A plane, or plain, is an even level surface. 
Let us suppose a smooth thin solid plane cutting the sun 
through the centre, extending out as far as the fixed stars, 
and terminating in a circle which passes through the mid¬ 
dle of the zodiac; in this plane the earth would move in 
its revolution round the sun; it is therefore called the plane 
of the earth’s orbit, and the circle in which this plane cuts 
the signs of the zodiac is the ecliptic. Let the fig. 1. plate 
IX. represent such a plane, S the sun, E the earth with its 
orbit, and A B C D the ecliptic passing through the middle 
of the zodiac. 

JEmily. If the ecliptic relates only to the heavens, why 
is it described upon the terrestrial globe? 

Airs. B. It is convenient for the demonstration of a 
variety of problems in the use of the globes; and besides, 
the obliquity of this circle to the equator is rendered more 
conspicuous by its being described on the same globe; and 
the obliquity of the ecliptic shows the inclination of the 
earth’s axis to the plane of its orbit. But to return to fig. 
2. plate VIII. 

The spaces between the several parallel circles on the 
terrestrial globe are called zones: that which is compre¬ 
hended between the tropics is distinguished by the name of 
the torrid zone; the spaces which extend from the tropics 
to the polar circles, the north and south temperate zones; 
and the spaces contained within the polar circles, the frigid 
zones. 

The several lines which, you observe, are drawn from 


rJLATK XV 





































































































































































































































+ 































































ON THE EARTH. 


105 


one pole to the other, cutting the equator at right angles, 
are called meridians. When any one of these meridians is 
exactly opposite to the sun, it is mid-day, or twelve o’clock 
in the day, with all the places situated on that meridian; 
and, with the places situated on the opposite meridian, it is 
consequently midnight. 

Emily. To places situated equally distant from these 
two meridians, it must then be six o’clock. 

Mrs. B. Yes; if they are to the east of the sun’s meri¬ 
dian it is six o’clock in the afternoon, because the sun will 
have previously passed over them; if to the west, it is six 
o’clock in the morning, and the sun will be proceeding to¬ 
wards that meridian. 

These circles which divide the globe into two equal parts, 
such as the equator and the ecliptic, are called greater cir¬ 
cles; to distinguish them from those which divide it into 
two unequal parts, as the tropics and polar circles, which 
are called lesser circles. AH circles are divided into 360 
equal parts, called degrees, and degrees into 60 equal parts, 
called minutes. The diameter of a circle is a right line 
drawn across it, and passing through the centre; for instance, 
the boundary of this sphere is a circle, and its axis the dia¬ 
meter of that circle; the diameter is equal to a little less 
than one-third of the circumference. Can you tell me near¬ 
ly how many degrees it contains? 

Caroline. It must be something less than one-third of 
360 degrees, or nearly 120 degrees. 

Mrs. B. Right; now Emily, you may tell me exactly 
how many degrees are contained in a meridian? 

Emily. A meridian, reaching from one pole to the other, 
is half a circle, and must therefore contain 180 degrees. 

Mrs. B. Very well; and what number of degrees are 
there from the equator to the poles? 

Caroline. The equator being equally distant from either 
pole, that distance must be half of a meridian, or a quarter 
of a circumference of a circle, and contain 90 degrees. 

Mrs. B. Besides the usual division of circles into de¬ 
grees, the ecliptic is divided into twelve equal parts, called 
signs, which bear the name of the constellations through 


106 


ON THE EARTH. 


which this circle passes in the heavens. The degrees mea¬ 
sured on the meridians from north to south, or south to 
north, are called degrees of latitude; those measured from 
east to west on the equator, the ecliptic, or any of the less¬ 
er circles, are called degrees of longitude; hence these cir¬ 
cles bear the name of longitudinal circles; they are also 
called parallels of latitude. 

Emily . The degrees of longitude must then vary in 
length, according to the dimensions of the circle on which 
they are reckoned; those, for instance, at the polar circles 
will be considerably smaller than those at the equator? 

Mrs. B. Certainly; since the degrees of circles of dif¬ 
ferent dimensions do not vary in number, they must neces¬ 
sarily vary in length. The degrees of latitude you may ob¬ 
serve, never vary in length; for the meridians on which they 
are reckoned are all of the same dimensions. 

Emily. And of what length is a degree of latitude? 

Mrs. B. Sixty geographical miles, which is equal to 
69£ English statute miles. 

Emily. The degrees of longitude at the equator must 
then be of the same dimensions? 

Mrs . B. They would, were the earth a perfect sphere; 
but its form is not exactly spherical, being somewhat protu¬ 
berant about the equator, and flattened towards the poles. 
This form is supposed to proceed from the superior action 
of the centrifugal power at the equator. 

Caroline. I thought I had understood the centrifugal 
force perfectly, but I do not comprehend its effects in this 
instance. 

Mrs. B. You know that the revolution of the earth on 
its axis must give every particle a tendency to fly off from 
the centre, that this tendency is stronger or weaker in pro¬ 
portion to the velocity with which the particle moves; now 
a particle situated near one of the polar circles makes one 
rotation in the same space of time as a particle at the equa¬ 
tor; the latter, therefore, having a much larger circle to de¬ 
scribe, travels proportionally faster, consequently the cen¬ 
trifugal force is much stronger at the equator than at the 
polar circles: it gradually decreases as you leave the equa- 


ON THE EARTH. 


107 ' 


of weight could be ascertained; for if the body under trial 
tor and approach the poles, where, as their is no rotatory 
motion, it entirely ceases. Supposing, therefore, the earth 
to have been originally in a fluid state, the particles in the 
torrid zone would recede much farther from the centre than 
those in the frigid zones; thus the polar regions would be¬ 
come flattened, and those about the equator elevated. 

Caroline. I did not consider that the particles in the 
neighbourhood of the equator move with greater velocity 
than those about the poles; this was the reason I could not 
understand you. 

Mrs. B. You must be careful to remember that those 
parts of a body which are farthest from the centre of mo¬ 
tion must move with the greatest velocity: the axis of the 
earth is the centre of its diurnal motion, and the equatorial 
regions the parts most distant from the axis. 

Caroline. My head then moves faster than my feet; and 
upon the summit of a mountain we are carried round quicker 
than in a valley? 

Mrs. B. Certainly; your head is more distant from the 
centre of motion than your feet; the mountain-top than the 
valley; and the more distant any part of a body is from the 
centre of motion, the larger is the circle it will describe, 
and the greater therefore mnst be its velocity. 

Emily. I have been reflecting, that if the earth is not a 
perfect circle— 

Mrs. B. A sphere you mean, my dear: a circle is a 
round line, every part of which is equally distant from the 
centre; a sphere or globe is a round body, the surface of 
which is every where equally distant from the centre. 

Emily. If, then, the earth is not a perfect sphere, but 
prominent at the equator, and depressed at the poles, would 
not a body weigh heavier at the equator than at the poles? 
For the earth being thicker at the equator, the attraction of 
gravity perpendicularly downwards must be stronger. 

Mrs. B. Your reasoning has some plausibility, but I 
am sorry to be obliged to add, that it is quite erroneous; for 
the nearer any part of the surface of a body is to the centre 
of attraction, the more strongly it is attracted; because the 


108 


ON THE EARTH. 


most considerable quantity of matter is about that centre. In 
regard to its effects, you might consider the power of gravity, 
as that of a magnet placed at the centre of attraction. 

Emily. But where you to penetrate deep into the earth, 
would gravity increase as you approached the centre? 

Mrs. B. Certainly not; I am referring only to any situa¬ 
tion on the surface of the earth. Were you to penetrate 
into the interior, the attraction of the parts above you would 
counteract that of the parts beneath you, and consequently 
diminish the power of gravity in proportion as you approach 
the centre; and if you reached that point, being equally at¬ 
tracted by the parts all around you, gravity would cease, 
and you would be without weight. 

Emily. Bodies, then, should weigh less at the equator 
than at the poles, since they are more distant from the cen¬ 
tre of gravity in the former than in the latter situation? 

Mrs. B. And this is really the case; but the difference 
of weight would be scarcely sensible, were it not augment¬ 
ed by another circumstance. 

Caroline. And what is this singular circumstance, which 
seems to disturb the laws of nature? 

Mrs. B. One that you are well acquainted with, as 
conducing more to the preservation than the destruction of 
order,—the centrifugal force. This we have just observed 
to be stronger at the equator; and as it tends to drive bodies 
from the centre, it is necessarily opposed to, and must lessen 
the power of gravity, which attracts them towards the cen¬ 
tre. We accordingly find that bodies weigh lightest at the 
equator, where the centrifugal force is greatest; and heavi¬ 
est at the poles, where this power is least. 

Caroline. Has the experiment been made in these dif¬ 
ferent situations? 

Mrs. B. Louis XIV. of France, sent philosophers both 
to the equator and to Lapland for this purpose: the severity 
of the climate, and obstruction of the ice, has hitherto ren¬ 
dered every attempt to reach the pole abortive; but the dif¬ 
ference of gravity at the equator and in Lapland is very 
perceptible. 

Caroline . Yet I do not comprehend how the difference 


ON THE EARTH. 


109 


decreased in weight, the weight which was opposed to it in 
the opposite scale must have diminished in the same pro¬ 
portion. For instance, if a pound of sugar did not weigh 
so heavy at the equator as at the poles, the leaden pound 
which served to weigh it, would not be so heavy either; 
therefore they would still balance each other, and the dif¬ 
ferent forceof gravity could not be ascertained by this means. 

Mrs. B. Your observation is perfectly just: the differ¬ 
ence of gravity of bodies situated at the poles and at the 
equator can not be ascertained by weighing them; a pendu¬ 
lum was therefore used for that purpose. 

Caroline. What, the pendulum of a clock? how could 
that answer the purpose? 

Mrs. B. A pendulum consists of aline, or rod, to one 
end of which a weight is attached, and it is suspended by 
the other to a fixed point, about which it is made to vibrate. 
Without being put in motion, a pendulum, like a plumb 
line, hangs perpendicular to the general surface of the earth, 
by which it is attracted; but if you raise a pendulum, gra > 
vity will bring it back to its perpendicular position. It will, 
however, not remain stationary there, for the velocity it has 
received during its descent, will impel it onwards, and it 
will rise on the opposite side to an equal height; from thence 
it is brought back by gravity, and again driven by the im¬ 
pulse of its velocity. 

Caroline. If so, the motion of a pendulum would be per¬ 
petual, and I thought you said, that there was no perpetual 
motion on the earth. 

Mrs. B. The motion of a pendulum is opposed by the 
resistance of the air in which it vibrates, and by the fric¬ 
tion of the part by which it is suspended: were it possible 
to remove these obstacles, the motion of a pendulum would 
be perpetual, and its vibrations perfectly regular; being of 
equal distances, and performed in equal times. 

Emily. That is the natural result of the uniformity of 
the power which produces these vibrations, for the force of 
gravity being always the same, the velocity of the pendulum 
must consequently be uniform. 

Caroline. No, Emily, you are mistaken; the cause is 
10 * 


110 


ON THE EARTH. 


not always uniform, and therefore the effect will not be so 
either. I have discovered it, Mrs. B.; since the force of 
gravity is less at the equator than at the poles, the vibra¬ 
tions of the pendulum will be slower at the equator than at 
the poles. 

Mrs. B. You are perfectly right, Caroline; it was by 
this means that the difference of gravity was discovered, and 
the true figure of the earth ascertained. 

Emily. But how do they contrive to regulate their time 
in the equatorial and polar regions? for, since in this part 
of the earth the pendulum of a clock vibrates exactly once 
in a second, if it vibrates faster at the poles and slower at 
the equator, the inhabitants must regulate their clocks in a 
different manner from ours. 

Mrs. B. The only alteration required is to lengthen 
the pendulum in one case, and to shorten it in the other; for 
the velocity of the vibrations of a pendulum depends on its 
length; and when it is said that a pendulum vibrates quicker 
at the pole than at the equator, it is supposing it to be of the 
same length. A pendulum which vibrates a second in this 
latitude is 36^ inches long. In order to vibrate at the equa¬ 
tor in the same space of time, it must be lengthened by the 
addition of a few lines; and at the poles it must be propor¬ 
tionally shortened. 

I shall now, I think, be able to explain to you the varia¬ 
tion of the seasons, and the difference of the length of the 
days and nights in those seasons; both effects resulting from 
the same cause. 

In moving round the sun, the axis of the earth is not per¬ 
pendicular to the plane of its orbit. Supposing this round 
table to represent the plane of the earth’s orbit, and this 
little globe, which has a wire passing through it, represent¬ 
ing the axis and poles, we shall call the earth; in moving 
round the table, the wire is not perpendicular to it, but 
oblique. 

Emily. Yes, I understand the earth does not go round 
the sun in an upright position, its axis is slanting or oblique. 

Mrs. B. All the lines, which you learnt in your last 
lesson, are delineated on this little globe; you must consi- 


ON THE EARTH. 


Ill 


der the ecliptic as representing the plane of the earth’s or¬ 
bit, and the equator, which crosses the ecliptic in two 
places, shows the degree of obliquity of the axis of the 
earth in that orbit, which is exactly 23^ degrees. The 
points in which the ecliptic intersects the equator are call¬ 
ed nodes. 

But I believe I shall make this clearer to you by revolv¬ 
ing the little globe round a candle, which shall represent 
the sun. (Plate IX. fig. 2.) 

As I now hold it, at A, you see it in the situation in 
which it is in the midst of summer, or what is called the 
summer solstice, which is on the 21st of June. 

Emily. You hold the wire awry, I suppose, in order to 
show that the axis of the earth is not upright? 

Mrs. B. Yes; in summer, the north pole is inclined 
towards the sun. In this season, therefore, the northern 
hemisphere enjoys much more of his rays than the southern. 
The sun, you see, now shines over the whole of the north 
frigid zone, and notwithstanding the earth’s diurnal revo¬ 
lution, which I imitate by twirling the ball on the wire, it 
will continue to shine upon it as long as it remains in this 
situation, whilst the south frigid zone is at the same time 
completely in obscurity. 

Caroline. That is very strange; I never before heard 
that there was constant day or night in any part of the 
world! How much happier the inhabitants of the north 
frigid zone must be than those of the southern; the first 
enjoy uninterrupted day, while the last are involved in per¬ 
petual darkness. 

Mrs. B. You judge with too much precipitation; exa¬ 
mine a little further, and you will find, that the two frigid 
zones share an equal fate. 

We shall now make the earth set off from its position in 
the summer solstice, and carry it round the sun; observe 
that the pole is always inclined in the same direction, and 
points to the same spot in the heavens. There is a fixed 
star situated near that spot, which is hence called the north 
polar star. Now let us stop the earth at B, and examine 
it in its present situation; it has gone through one quarter 


112 


ON THE EARTH. 


of its orbit, and is arrived at that point at which the eclip¬ 
tic cuts or crosses the equator, and which is called the au¬ 
tumnal equinox. 

Emily. That is then one of the nodes. 

The sun now shines from one pole to the other, just as 
it would constantly do, if the axis of the earth were per¬ 
pendicular to its orbit. 

Mrs. B. Because the inclination of the axis is now 
neither towards the sun nor in the contrary direction; at 
this period of the year, therefore, the days and nights are 
equal in every part of the earth. But the next step she 
takes in her orbit, you see, involves the north pole in dark¬ 
ness, whilst it illumines that of the south; this change was 
gradually preparing as I moved the earth from summer to 
autumn; the arctic circle, which was at first entirely illu¬ 
mined, began to have short nights, which increased as the 
earth approached the autumnal equinox; and the instant it 
passed that point, the long night of the north pole com¬ 
mences, and the south sole begins to enjoy the light of the 
sun. We shall now make the earth proceed in its orbit, 
and you may observe that as it advances, the days shorten 
and the nights lengthen, throughout the northern hemi¬ 
sphere, until it arrives at the winter solstice, on the 21st of 
December, when the north frigid zone is entirely in dark¬ 
ness, and the southern has uninterrupted day-light. 

Caroline. Then, alter all, the sun which I thought so 
partial, confers his favours equally on all. 

Mrs. B. Not so neither: the inhabitants of the torrid 
zone have much more heat than we have, as the sun’s rays 
fall perpendicularly on them, while they shine obliquely on 
the rest of the world, and almost horizontally on the poles; 
for during their long day of six months, the sun moves round 
their horizon without either rising or setting; the only ob¬ 
servable difference, is that it is more elevated by a few de¬ 
grees at mid-day, than at mid-night. 

Emily. To a person placed in the temperate zone, in 
the situation in which we are in England, the sun will shine 
neither so obliquely as it does on the poles, nor so vertically 


ON THE EARTH. 113 

as at the equator; but its rays will fall upon him more ob¬ 
liquely in autumn and winter, than in summer. 

Caroline. And therefore, the inhabitants of the temper¬ 
ate zones will not have merely one day and one night in 
the year as happens at the poles, nor will they have equal 
days and equal nights as at the equator; but their days and 
nights will vary in length, at different times of the year, 
according as their respective poles incline towards or from 
the sun, and the difference will be greater in proportion to 
their distance from the equator. 

Mrs. B. We shall now follow the earth through the 
other half of her orbit, and you will observe, that now ex¬ 
actly the same effect takes place in the southern hemisphere 
as what we have just remarked in the northern. Day com¬ 
mences at the south pole when night sets in at the north 
pole; and in every other part of the southern hemisphere 
the days are longer than the nights, while, on the contrary, 
our nights are longer than our days. When the earth ar¬ 
rives at the vernal equinox, D, where the ecliptic again 
cuts the equator, on the 25th of March, she is situated, 
with respect to the sun, exactly in the same position, as in 
the autumnal equinox; and the only difference with respect 
to the earth, is, that it is now autumn in the southern hemi¬ 
sphere, whilst it is spring with us. 

Caroline. Then the days and nights are again every 
where equal? 

Mrs. B. Yes, for the half of the globe which is enlight¬ 
ened, extends exactly from one pole to the other, the sun 
rises to the north pole, and the sun sets to the south pole; 
but in every other part of the globe, the day and night is 
of twelve hours length, hence the word equinox, which is 
derived from the Latin, meaning equal night. 

As the earth proceeds towards summer, the days lengthen 
in the northern hemisphere, and shorten in the southern, 
till the earth reaches the summer solstice, when the north 
frigid zone is entirely illumined, and the southern is in com¬ 
plete darkness; and we have now brought the earth again 
to the spot from whence we first accompanied her. 

Emily . This is indeed a most satisfactory explanation 


114 


ON THE EARTH, 


of the seasons; and the more I learn, the more I admire 
the simplicity of means by which such wonderful effects 
are produced. 

Mrs. B. I know not which is most worthy of our ad¬ 
miration, the cause, or the effect of the earth’s revolution 
round the sun. The mind can find no object of contem¬ 
plation more sublime, than the course of this magnificent 
globe, impelled by the combined powers of projection and 
attraction to roll in one invariable course around the source 
of light and heat: and what can be more delightful than the 
beneficent effects of this vivifying power on its attendant 
planet. It is at once the grand principle which animates 
and fecundates nature. 

Emily. There is one circumstance in which this little 
ivory globe appears to me to differ from the earth; it is not 
quite dark on that side of it which is turned from the can¬ 
dle, as is the case with the earth when neither moon nor 
stars are visible. 

Mrs. B. This is owing to the light of the candle being 
reflected by the walls of the room on every part of the globe, 
consequently that side of the globe on which the candle 
does not directly shine, is not in total darkness. Now the 
skies have no walls to reflect the sun’s light on that side of 
our earth which is in darkness. 

Caroline. I beg your pardon, Mrs. B., I think that the 
moon and stars answer the purpose of walls in reflecting 
the sun’s light to us in the night. 

Mrs. B. Very well, Caroline; that is to say, the moon 
and planets; for the fixed stars, you know, shine by their 
own light. 

Emily. You say, that the superior heat of the equatorial 
parts of the earth, arises from the rays falling perpendicu¬ 
larly on those regions, whilst they fall obliquely on these 
more northern regions; now I do not understand why per¬ 
pendicular rays should afford more heat than oblique rays. 

Caroline. You need only hold your hand perpendicu¬ 
larly over the candle, and then hold it sideways obliquely, 
to be sensible of the difference. 


i 


1*1, ATK A . 


r 








_ 



































©N THE EARTH. 115 

Emily. I do not doubt the fact, but I wish to have it ex¬ 
plained. 

Mrs. B. You are quite right; if Caroline had not been 
satisfied with ascertaining the fact, without understanding 
it, she would not have brought forward the candle as an 
illustration; the reason why you feel so much more heat if 
you hold your hand perpendicularly over the candle, than 
if you hold it sideways, is because a stream of heated va¬ 
pour constantly ascends from the candle, or any other burn¬ 
ing body, which being lighter than the air of the room, does 
not spread laterally but rises perpendicularly, and this led 
you to suppose that the rays were hotter in the latter direc¬ 
tion. Had you reflected, you would have discovered that 
rays issuing from the candle sideways, are no less perpen¬ 
dicular to your hand when held opposite to them, than the 
rays which ascend when your hand is held over them. 

The reason why the sun’s rays afford less heat when in 
an oblique direction than when perpendicular, is because 
fewer of them fall upon an equal portion of the earth; this 
will be understood better by referring to plate (X. fig. 1.,) 
which represents two equal portions of the sun’s rays, shin¬ 
ing upon different parts of the earth. Here it is evident, 
that the same quantity of rays fall on the space A B, as fall 
on the space B C; and as A B is less than B C, the heat 
and light will be much stronger in the former than*in the 
latter; A B,you see, represents the equatorial regions, where 
the sun shines perpendicularly; and B C,the temperate and 
frozen climates, where his rays fall more obliquely. 

Emily. This accounts not only for the greater heat of 
the equatorial regions, but for the greater heat of summer; 
as the sun shines less obliquely in summer than in winter. 

Mrs. B. This you will see exemplified in figure 2, in 
which the earth is represented, as it is situated on the 21st 
of June, and England receives less oblique and consequently 
a greater number of rays, than at any other season; and 
figure 3, shows the situation of England on the 21st of De¬ 
cember, when the rays of the sun fall most obliquely upon 
her. But there is also another reason why oblique rays 
give less heat, than perpendicular rays; which is, that they 


116 


ON THE EARTH. 


have a greater portion of the atmosphere to traverse; and 
though it is true, that the atmosphere is itself a transparent 
bodj, Creel) admitting the passage of the sun’s rays, yet it 
is always loaded more or less with dense and foggy vapour, 
which the rays of the sun can not easily penetrate; there¬ 
fore the greater the quantity of atmosphere the sun’s rays 
have to pass through in their way to the earth, the less heat 
they will retain when they reach it. This will be better 
understood, by referring to (fig. 4.) The dotted line round 
the earth, describes the extent of the atmosphere, and the 
lines which proceed from the sun to the earth, the passage 
of two equal portions of the sun’s rays to the equatorial and 
polar regions; the latter you see, from its greater obliquity 
passes through a greater extent of atmosphere. 

Caroline. And this, no doubt, is the reason why the sun 
in the morning and the evening gives so much less heat, 
than at mid-day. 

Mrs. B. The diminution of heat, morning and evening, 
is certainly owing to the greater obliquity of the sun’s rays; 
and as such they are affected by both the causes, which I 
have just explained to you; the difficulty of passing through 
a foggy atmosphere is perhaps more particularly applicable 
to them, as mist and vapours are prevalent about the time 
of sunrise and sunset.. But the diminished obliquity of the 
sun’s rays, is not the sole cause of the heat of summer; the 
length of the days greatly conduces to it; for the longer the 
sun is above the horizon, the more heat he will communi¬ 
cate to the earth. 

Caroline. Both the longest days, and the most perpen¬ 
dicular rays, are on the 21st of June; and yet the greatest 
heat prevails in July and August. 

Mrs. B. Those parts of the earth which are once heat¬ 
ed, retain the heat for some length of time, and the addi¬ 
tional heat they receive, occasions an elevation of tempera¬ 
ture, although the days begin to shorten, and the sun’s rays 
to fall more obliquely. For the same reason, we have 
generally more heat at three o’clock in the afternoon, than 
at twelve when the sun is on the meridian. 


ON THE EARTH. 


117 


Emily. And pray, have the other planets the same vi¬ 
cissitudes of seasons, as the earth ? 

Mrs. B. Some of them more, some less, according as 
their axes deviate more or less from the perpendicular to 
the plane of their orbits. The axis of Jupiter is nearly 
perpendicular to the plane of his orbit; the axes of Mars 
and of Saturn are each inclined at angles of about sixty de¬ 
grees; whilst the axis of Venus is believed to be elevated 
only fifteen or twenty degrees above her orbit; the vicissi¬ 
tudes of her seasons must therefore be considerably greater 
than ours. For further particulars respecting the planets, 
I shall refer you to Bonnycastle’s Introduction to Astronomy. 

I have but one more observation to make to you relative 
to the earth’s motion, which is, that although we have but 
365 days and nights in the year, she performs 366 com¬ 
plete revolutions on her axis during that time. 

Caroline. How is that possible? for every complete re¬ 
volution must bring the same place back to the sun. It is 
now just twelve o’clock, the sun is, therefore, on our meri¬ 
dian; in twenty-four hours will it not be returned to our 
meridian again, and will not the earth have made a com¬ 
plete rotation on its axis? 

Mrs. B. If the earth had no progressive motion in its 
orbit whilst it revolves on its axis, this would be the case; 
but as it advances almost a degree westward in its orbit, in 
the same time that it completes a revolution eastward on its 
axis, it must revolve nearly one degree more in order to 
bring the same meridian back to the sun. 

Caroline. Oh, yes! it will require as much more of a 
second revolution to bring the same meridian back to the 
sun, as is equal to the space the earth has advanced in her 
orbit, that is, nearly a degree; this difference is, however, 
very little. 

Mrs. B. These small daily portions of rotation are 
each equal to the three hundred and sixty-fifth part of a 
circle, which at the end of the year amounts to one com¬ 
plete rotation. 

Emily. That is extremely curious. If the earth then, 

11 


118 


ON THE EARTH. 


liad no other than its diurnal motion, we should have 366 
days in the year. 

Mrs. B. We should have 366 days in the same period 
of time that we now have 365; but if we did not revolve 
round the sun, we should have no natural means of com¬ 
puting years. 

You will be surprised to hear, that if time is calculated 
by the stars instead of the sun, the irregularity which we 
have jus* noticed does not occur, and that one complete ro¬ 
tation of the earth on its axis, brings the same meridian 
back to any fixed star. 

Emily. That seems quite unaccountable; for the earth 
advances in her orbit with regard to the fixed stars, the same 
as with regard to the sun. 

Mrs, B. True, but then the distance of the fixed stars 
is so immense, that our solar system is in comparison to it 
but a spot, and the whole extent of the earth’s orbit but a 
point; therefore, whether the earth remain stationary, or 
whether it revolved in its orbit during its rotation on its 
axis, no sensible difference would be produced with regard 
to the fixed stars. One complete revolution brings the same 
meridian back to the same fixed star; hence the fixed stars 
appear to go round the earth in a shorter time than the sun 
by three minutes fifty-six seconds of time. 

Caroline . These three minutes fifty-six seconds is the 
time which the earth takes to perform the additional three 
hundred and sixty fifth part of the circle, in order to bring 
the same meridian back to the sun. 

Mrs. B. Precisely. Hence the stars gain every day 
three minutes fifty-six seconds on the sun, which makes 
them rise that portion of time earlier every day. 

When time is calculated by the stars it is called sidereal 
time, when by the sun solar or apparent time. 

Caroline. Then a sidereal day is three minutes fifty-six 
seconds shorter than a solar day of twenty-four hours. 

Mrs. B. I must also explain to you what is meant by a 
sidereal year. 

The common year, called the solar or tropical year, con¬ 
taining 365 days, five hours, forty-eight minutes and fifty- 





1‘l.AJ'l. XT 








































ON THE EARTH. 


119 


two seconds, is measured from the time the sun sets out 
from one of the equinoxes, or solstices, till it returns to the 
same again; but this year is completed before the earth has 
finished one entire revolution in its orbit. 

Emily. I thought that the earth performed one complete 
revolution in its orbit every year; what is the reason of this 
variation? 

Mrs. B . It is owing to the spheroidal figure of the earth. 
The elevation about the equator produces much the same 
effect as if a similar mass of matter, collected in the form 
of a moon, revolved round the equator. When this moon 
acted on the earth in conjunction with or in opposition to 
the sun, variations in the earth’s motion would be occasion¬ 
ed, and these variations produce what is called the preces¬ 
sion of the equinoxes. 

Emily. What does that mean? I thought the equinoctial 
points, or nodes, were fixed points in the heavens, in which 
the equator cuts the ecliptic. 

Mrs. B. These points are not quite fixed, but have an 
apparently retrograde motion, that is to say, instead of be¬ 
ing every revolution in the same place, they move back¬ 
wards. Thus if the vernal equinox is at A, (fig. 1. plate 
XI.) the autumnal one will be at B instead of C, and the 
following vernal equinox at D instead of at A, as would be 
the case if the equinoxes were stationary at opposite points 
of the earth’s orbit. 

Caroline. So that when the earth moves from one equi¬ 
nox to the other, though it takes half a year to perform the 
journey, it has not travelled through half its orbit. 

Mrs. B, And, consequently, when it returns again to 
the first equinox, it has not completed the whole of its or¬ 
bit. In order to ascertain when the earth has performed 
an entire revolution in its orbit, we must observe when the 
sun returns in conjunction with any fixed star; and this is 
called a sidereal year. Supposing a fixed star situated at 
E, (fig. 1. plate XI.) the sun would not appear in conjunc¬ 
tion with it till the earth had returned to A, when it would 
have completed its orbit. 


120 


ON THE EARTH. 


Emily. And how much longer is the sidereal than the 
solar year? 

Mrs. B. Only twenty minutes; so that the variation of 
the equinoctial points is very considerable. I have given 
them a greater extent in the figure in order to render them 
sensible. 

In regard to time, I must further add, that the earth’s 
diurnal motion on an inclined axis, together with its annual 
revolution in an elliptic orbit, occasions so much complica¬ 
tion in its motion, as to produce many irregularities; there¬ 
fore true equal time can not be measured by the sun. A 
clock, which was always perfectly correct, would in some 
parts of the year be before the sun, and in other parts after 
it. There are but four periods in which the sun and a per¬ 
fect clock would agree, which is the 16th of April, the 16th 
of June, the 23d of August, and the 24th of December. 

Emily. And is there any considerable difference between 
solar time and true time? 

Mrs. B. The greatest difference amounts to between 
fifteen and sixteen minutes. Tables of equation are con¬ 
structed for the purpose of pointing out and correcting these 
differences between solar time and equal or mean time, 
which is the denomination given by astronomers to true 
time. 


CONVERSATION IX. 


ON THE MOON. 


OF THE MOON’S MOTION.—PHASES OF THE MOON.—ECLIPSES OF THE MOON. 

—eclipses of jupiteh’s moons.—of the latitude and longitude. 

-OF THE TRANSITS OF THE INFERIOR PLANETS.—OF THE TIDES. 


MRS. B. 

We shall to-day confine our attention to the moon, which 
offers many interesting phenomena. 

The moon revolves round the earth in the space of about 
twenty-nine days and a half, in an orbit nearly parallel to 
that of the earth, and accompanies us in our revolution 
round the sun. 

Emily . Her motion then must be rather of a complica¬ 
ted nature; for as the earth is not stationary, but advances 
in her orbit whilst the moon goes round her, the moon must 
proceed in a sort of progressive circle. 

Mrs. B. That is true; and there are also other circum¬ 
stances which interfere with the simplicity and regularity 
of the moon’s motion, but which are too intricate for you 
to understand at present. 

The moon always presents the same face to us, by which 
it is evident that she turns but once upon her axis, while 
she performs a revolution round the earth; so that the in¬ 
habitants of the moon have but one day and one night in 
the course of a lunar month. 

Caroline . We afford them, however, the advantage of a 
magnificent moon to enlighten their long nights. 


122 


ON THE MOON. 


Mrs. B. That advantage is but partial; for since we al¬ 
ways see the same hemisphere of the moon, the inhabitants 
of that hemisphere alone can perceive us. 

Caroline. One half of the moon then enjoys our light 
every night, while the other half has constantly nights of 
darkness. If there are any astronomers in those regions, 
they would doubtless be tempted to visit the other hemis¬ 
phere, in order to behold so grand a luminary as we must 
appear to them. But, pray, do they see the earth under all 
the changes which the moon exhibits to us? 

Airs. B. Exactly so. These changes are called the phases 
of the moon, and require some explanation. In fig. 2, plate 
XI. let us say that S represents the sun, E the earth, and 
A B C D the moon in different parts of her orbit. When 
the moon is at A, her dark side being turned towards the 
earth, we shall not see her as at a; but her disappearance 
is of very short duration, and as she advances in her orbit, 
we perceive her under the form of a new moon: when she 
has gone through one eighth of her orbit at B, one quarter 
of her enlightened hemisphere will be turned towards the 
earth, and she will then appear horned as at b; when she 
has performed one quarter of her orbit, she shows us one 
half of her enlightened side as at c; at d she is said to be 
gibbous, and at e the whole of the enlightened side appears 
to us, and the moon is at full. As she proceeds in her or¬ 
bit she becomes again gibbous, and her enlightened hemis¬ 
phere turns gradually away from us until she completes her 
orbit and disappears, and then again resumes her form of a 
new moon. 

When the moon is at full, or a new moon, she is said to 
be in conjunction with the sun, as they are then both in the 
same direction with regard to the earth; when at her quar¬ 
ters she is said to be in opposition to the sun. 

Emily. Are not the eclipses produced by the moon pass¬ 
ing between the sun and the earth? 

Mrs. B. Yes; when the moon passes between the sun 
and the earth, she intercepts his rays, or, in other words, 
casts a shadow on the earth, then the sun is eclipsed, and 












































































































































ON THE MOON. 123 

the day light gives place to darkness, while the moon’s 
shadow is passing over us. 

When, on the contrary, the earth is between the sun and 
the moon, it is we who intercept the sun’s rays, and cast a 
shadow on the moon; the moon in then darkened, she dis¬ 
appears from our view, and is eclipsed. 

Emily . But as the moon goes round the earth every 
month, she must be once during that time between the earth 
and the sun, and the earth must likewise be once between 
the sun and the moon, and yet we have not a solar and a lu¬ 
nar eclipse every month? 

Mrs . B. The orbits of the earth and moon are not exact¬ 
ly parallel, but cross or intersect each other; and the moon 
generally passes either above or below the earth when she 
is in conjunction with the sun, and does not therefore inter¬ 
cept the suu’s rays, and produce an eclipse; for this can 
take place only when the earth and moon are in conjunc¬ 
tion in that part of their orbits which cross each other, (call¬ 
ed the nodes of their orbits) because it is then only, that 
they are both in a right line with the sun. 

Emily. And a partial eclipse takes place, I suppose, 
when the moon, in passing by the earth, is not sufficiently 
above or below the earth’s shadow entirely to escape it? 

Mrs . B. Yes, one edge of her disk then dips into the 
shadow, and is eclipsed; but as the earth is larger than the 
moon, when the eclipse happens precisely at the nodes, they 
are not only total, but last for some length of time. 

When the sun is eclipsed, the total darkness is confined 
to one particular part of the earth, evidently showing that 
the moon is smaller than the earth, since she can not entire¬ 
ly screen it from the sun. In fig. 1, pi. XII. you will find 
a solar eclipse described; S is the sun, M the moon, and 
E the earth; and the moon’s shadow, you see, is not large 
enough to cover the earth. The lunar eclipses, on the con¬ 
trary, are visible from every part of the earth, where the 
moon is above the horizon; and we discover by the length 
of time which the moon is passing through the earth’s shadow, 
that it would be sufficient io eclipse her totally, were she 47 


m 


ON THE MOON. 


times her actual size; it follows therefore that the earth is 
47 times the size of the moon. 

In fig. 2, S represents the sun, which pours forth rays of 
light in straight lines in every direction. E is the earth, 
and M the moon. Now a ray of light coming from one ex¬ 
tremity of the sun’s disk in the direction A B, will meet 
another coming from the opposite extremity, in the direc¬ 
tion C B; the shadow of the earth can not therefore extend 
beyond B; as the sun is larger than the earth, the shadow 
of the latter is conical, or the figure of a sugar loaf; it gra¬ 
dually diminishes, and is much smaller than the earth where 
the moon passes through it, and yet we find the moon to be 
not only totally eclipsed, but some length of time in dark¬ 
ness, and hence we are enabled to ascertain its real dimen¬ 
sions. 

Emily. When the moon eclipses the sun to us, we must 
be eclipsed to the moon? 

Mrs. B. Certainly; for if the moon intercepts the sun’s 
rays, and casts a shadow on us, we must necessarily disap¬ 
pear to the moon, but only partially, as in fig. 1. 

Caroline. There must be a great number of eclipses 
in the distant planets, which have so many moons? 

Mrs. B. Yes, few days pass without an eclipse taking 
place; for among the number of satellites, one or the other 
of them are continually passing either between their planet 
and the sun, or between the planet and each other. Astro¬ 
nomers are so well acquainted with the motion of the pla¬ 
nets and their satellites, that they have calculated not only 
the eclipses of our moon, but those of Jupiter, with such 
perfect accuracy, that it has afforded a means of ascertain¬ 
ing the longitude. 

Caroline. But is.it not very easy to find both the lati¬ 
tude and longitude of any place by a map or globe? 

Mrs. B. If you know where you are situated, there is no 
difficulty in ascertaining the latitude or longitude of the 
place by referring to a map; but supposing that you had 
been a length of time at sea, interrupted in your course by 
storms, a map would afford you very little assistance in dis¬ 
covering where you were. 


ON THE MOON. 


125 


Caroline. Under such circumstances, I confess I should 
be equally at a loss to discover either latitude or longitude. 

Mrs. B. The latitude may be easily found by taking 
the altitude of the pole; that is so say, the number of degrees 
that it is elevated above the horizon, for the pole appears 
more elevated as we approach it, and less as we recede 
from it. 

Caroline. But unless you can see the pole, how can you 
take its altitude? 

Mrs. B. The north pole points constantly towards one 
particular part of the heavens in which a star is situated, 
called the Polar Star: this star is visible on clear nights 
from every part of the northern hemisphere, the altitude of 
the polar star is therefore the same number of degrees as 
that of the pole; the latitude may also be determined by ob¬ 
servations made on the sun or any of the fixed stars: the 
situation therefore of a vessel at sea, with regard to 
north and south, is easily ascertained. The difficulty is re¬ 
specting east and west, that is to say, its longitude. As 
we have no eastern poles from which we can reckon our 
distance, some particular spot must be fixed upon for that 
purpose. The English reckon from the meridian of Green¬ 
wich, where the royal observatory is situated; in French 
maps you will find that the longitude is reckoned from Paris. 

The rotation of the earth on its axis in 24 hours from 
west to east, occasions, you know, an apparent motion of 
the sun and stars in a contrary direction, and the sun ap¬ 
pears to go round the earth in the space of 24 hours, pass¬ 
ing over fifteen degrees, or a twenty-fourth part of the 
earth’s circumference every hour; therefore, when it is twelve 
o’clock in London, it is one o’clock in any plaee situated fif¬ 
teen degrees to the east of London, as the sun must have 
passed the meridian of that place an hour before he reaches 
that of London. For the same reason it is eleven o’clock 
in any place situated fifteen degrees to the west of Lon¬ 
don, as the sun will not come to that meridian till an hour 
later. 

If then the captain of a vessel at sea, could know precise¬ 
ly what was the hour at London, he could, by looking at 


126 


ON THE MOON. 


his watch, and comparing it with the hour of the spot in 
which he was, ascertain the longitude. 

Emily But if he had not altered his watch, since he 
sailed from London, it would indicate the hour it was then 
in London. 

Mrs. B. True; but in order to know the hour of the 
day of the spot in which he is, the captain of a vessel regu¬ 
lates his watch by the sun when it reaches the meridian. 

Emily. Then if he had two watches, he might keep one 
regulated daily, and leave the other unaltered; the former 
would indicate the hour of the place in which he was situ¬ 
ated, and the latter the hour of London; and by comparing 
them together, he would be able to calculate his longitude. 

Mrs. B. You have discovered, Emily, a mode of find¬ 
ing the longitude, which I have the pleasure to tell you, is 
universally adopted: watches of a superior construction, 
called chronometers, or time-keepers, are used for this pur¬ 
pose; but the best watches are liable to imperfections, and 
should the time-keeper go too fast or too slow, there would 
be no means of ascertaining the error; implicit reliance can 
not consequently be placed upon them. 

Recourse is therefore had to the eclipses of Jupiter’s sa¬ 
tellites. A table is made of the precise time at which the 
several moons are eclipsed to a spectator at London; when 
they appear eclipsed to a spectator in any other spot, he 
may, by consulting the table, know what is the hour at 
London; for the eclipse is visible at the same moment from 
\yhatever place on the earth it is seen. He has then only 
tolook at the watch which points out the hour of the place 
in which he is, and by observing the difference of time 
there, and at London, he may immediately determine his 
longitude. 

Let us suppose, that a certain moon of Jupiter is always 
eclipsed at six o’clock in the evening; arid that a man at 
sea consults his watch, and finds that it is ten o’clock at 
night, where he is situated, at the moment the eclipse takes 
place; what will be his longitude? 

Emily. That is four hours later than in London: four 
times fifteen degrees make 60; he would, therefore, be sixty 


ON THE MOON. 127 

degrees east of London, for the sun must have passed his 
meridian before it reaches that of London. 

Mrs. B. For this reason the hour is always later than 
in London, when the place is east longitude, and earlier 
when it is west longitude. Thus the longitude can be as¬ 
certained whenever the eclipses of Jupiter’s moons are 
visible. 

But it is not only the secondary planets which produce 
eclipses, for the primary planets near the sun eclipse him 
to those at a greater distance when they come in conjunc¬ 
tion in the nodes of their orbits, but as the primary planets 
are much longer in performing their course round the sun, 
than the satellites in going round their primary planets, these 
eclipses very seldom occur. Mercury and Venus have how¬ 
ever passed in a right line between us and the sun, but be¬ 
ing at so great a distance from us, their shadows did not 
extend so far as the earth; no darkness was therefore pro¬ 
duced on any part of our globe; but the planet appeared like 
a small black spot, passing across the sun’s disk; this is 
called a transit of the planet. 

It was by the last transit of Venus, that astronomers 
were enabled to calculate with some degree of accuracy the 
distance of the earth from the sun, and the dimensions of 
the latter. 

Emily. I have heard that the tides are affected by the 
moon, but I can not conceive what influence it can have on 
them. 

Mrs. B. They are produced by the moon’s attraction, 
which draws up the waters in a protuberance. 

Caroline. Does attraction act on water more power¬ 
fully than on land? I should have thought it would have 
been just the contrary, for land is certainly a more dense 
body than water? 

Mrs. B Tides do not arise from water being more 
strongly attracted than land, for this certainly is not the 
case; but the cohesion of fluids being much less than that 
of solid bodies, they more easily yield to the power of gra¬ 
vity, in consequence of which, the waters immediately be¬ 
low the moon are drawn up by it in a protuberance, pro- 


m 


ON THE MOON. 


ducing a full tide, or what is commonly called high water, 
at the spot where it happens. So far the theory of the tides 
is not difficult to understand. 

Caroline. On the contrary, nothing can be more simple; 
the waters, in order to rise up under the moon, must draw 
the waters from the opposite side of the globe, and occasion 
ebb-tide, or low water in those parts. 

Mrs. B. You draw your conclusion rather too hastily, 
my dear; for according to your theory, we should have full 
tide only once in twenty-four hours, that is, every time that 
we were below the moon, while we find that we have two 
tides in the course of twenty-four hours, and that it is high 
water with us and with our antipodes at the same time. 

Caroline. Yet it must be impossible for the moon to 
attract the sea in opposite parts of the globe, and in opposite 
directions at the same time. 

Mrs. B. This opposite tide is rather more difficult to 
explain, than that which is drawn up beneath the moon; 
with a little attention, however, I hope I shall be able to 
make you understand it. 

You recollect that the earth and moon are mutually at¬ 
tracted towards a point, their common centre of gravity and 
of motion; can you tell me what it is that prevents their 
meeting and uniting at this point? 

Emily. Their projectile force, which gives them a ten¬ 
dency to fly from this centre. 

Mrs. B. And is hence called their centrifugal force. 
Now we know that the centrifugal force increases in pro¬ 
portion to the distance from the centre of motion. 

Caroline. Yes, I recollect your explaining that to us, 
and illustrating it by the motion of the flyers of a wind-mill, 
and the spinning of a top. 

Emily. And it was but the other day you showed us 
that bodies weighed less at the equator than in the polar 
regions, in consequence of the increased centrifugal force 
in the equatorial parts. 

Mrs. B. Very well. The power of attraction, on the 
contrary, increases as the distance from the centre of gravi¬ 
ty diminishes; when, therefore, the two centres of gravity 


ON THE'MOON. 


129 


and of motion are in the same spot, as is the case with re¬ 
gard to the moon and the earth, the centrifugal power and 
those of attraction, will be in inverse proportion to each 
other; that is to say, where the one is strongest, the other 
will be weakest. 

Emily. Those parts of the ocean, then, which are most 
strongly attracted will have least centrifugal force, and those 
parts which are least attracted, will have the greatest cen¬ 
trifugal force. 

Mrs. B. In order to render the question more simple, 
let us suppose the earth to be every where covered by the 
ocean, as represented in (fig. 3. pi. XII.) M is the moon, 
A B C D the earth, and X the common centre of gravity 
and of motion of these two planets. Now the waters on 
the surface of the earth, about A, being more strongly at¬ 
tracted than any other part, will be elevated: the attraction 
of the moon at R and C being less, and at D least of all. 
But the centrifugal force at D, will be greatest, and the 
waters there, will in consequence have the greatest tenden¬ 
cy to recede from the moon; the waters at B and C will 
have less, tendency to recede, and at A least of all. The 
waters, therefore, at D, will recede furthest, at the same 
time that they are least attracted, and in consequence will 
be elevated in a protuberance similar to that at A. 

Emily. The tide A, then, is produced by the moon’s at¬ 
traction, and increased by the feebleness of the centrifugal 
power in those parts; and the tide D is produced by the 
centrifugal force, and increased by the feebleness of the 
moon’s attraction in those parts. 

Caroline. And when it is high water at A and D, it is 
low water at B and C: now I think I comprehend the na¬ 
ture of the tides again, though I confess it is not quit so easy 
as I at first thought. 

But, Mrs. B., why does not the sun produce tides as well 
as the moon; tor its attraction is greater than that of the 
moon. 

Mrs. B. It would be at an equal distance, but our vi¬ 
cinity to ti e moon makes her influence more powerfuk The 
sun has however, a considerable effect on the tides, and in- 

12 


130 


ON THE MOON. 


creases or diminishes them as it acts in conjunction with, or 
in opposition to the moon. 

Emily. I do not quite understand that. 

Mrs. B. The moon is a month in going round the earth; 
twice during that time, therefore, at full and at change, she 
is in the same direction as the sun, both then act in con¬ 
junction on the earth, and produce very great tides, called 
spring tides, as described in fig. 4, at A and B; but when the 
moon is at the intermediate parts of her orbit, the sun, in¬ 
stead of affording assistance, weakens her power by acting 
in opposition to it; and smaller tides are produced, called 
neap tides, as represented in fig. 5. 

Emily. I have often observed the difference of these 
tides when I have been at the sea side. 

But since attraction is mutual between the moon and the 
earth, we must produce tides in the moon; and these must 
be more considerable in proportion as our planet is larger. 
And yet the moon does not appear of an oval form. 

Mrs. B. You must reccollect, that in order to render the 
explanation of the tides clearer, we suppose the whole sur¬ 
face of the earth to be covered with the ocean; but that is 
not really the case, either with the earth or the moon, and 
the land which intersects the water destroys the regularity 
of the effect. 

Caroline. True; we may, however be certain that when¬ 
ever it is high water the moon is immediately over our heads. 

Mrs. B. Not so either; for as a similar effect is pro¬ 
duced on that part of the globe immediately beneath the 
moon, and on that part most distant from it, it can not be 
over the heads of the inhabitants of both those situations at 
the same time. Besides, as the orbit of the moon is very 
nearly parallel to that of the earth, she is never vertical but 
to the inhabitants of the torrid zone; in that climate, there¬ 
fore, the tides are greatest, and they diminish as you recede 
from it and approach the poles.' 

Caroline. In the torrid zone, then, I hope you will grant 
that the moon is immediately over, or opposite the spots 
where it is high water? 

Mrs. B. I can not even admit that; for the ocean natu- 


ON THE MOON. 


131 


rally partaking of the earth’s motion, in its rotation from 
west to east, the moon, in forming a tide, has to contend 
against the eastern motion of the waves. All matter, you 
know, by its inertia, makes some resistance to a change of 
state; the waters, therefore, do not readily yield to the attrac¬ 
tion of the moon, and the effect of her influence is not com¬ 
plete till three hours after she has passed the meridian, 
where it is full tide. 

Emily. Pray what is the reason that the tide is three- 
quarters of an hour later every day? 

Mrs. B. Because it is twenty-four hours and three- 
quarters before the same meridian on our globe returns be¬ 
neath the moon. The earth revolves on its axis in about 
twenty-four hours; if the moon were stationary, therefore, 
the same part of our globe would, every twenty-four hours, 
return beneath the moon, but as during our daily revolution 
the moon advances in her orbit, the earth must make more 
than a complete rotation in order to bring the same meridian 
opposite the moon: we are three-quarters of an hour in over¬ 
taking her. The tides, therefore, are retarded for the same 
reason that the moon rises later by three-quarters of an hour 
every day. 

We have now, I think, concluded the observations I had 
to make to you on the subject of astronomy; at our next in¬ 
terview, I shall attempt to explain to you the elements of hy¬ 
drostatics. 


CONVERSATION X. 


ON THE MECHANICAL PROPERTIES OF FLUIDS. 


DEFINITION OF A FLUID.-DISTINCTION BETWEBN FLUIDS AND LIQ.UIDS.- 

OF NON-ELASTIC FLUIDS.-SCARCELY SUSCEPTIBLE OF COMPRESSION.- 

OF THE COHESION OF FLUIDS.-OF THEIR GRAVITATION.-OF THEIR 

EiiUlLfBRIUM.-OF THEIR PRESSURE.-OF SPECIFIC GRAVITY.-OF THE 

SPECIFIC GRAVITY OF BODIES HEAVIER THAN WATER.-OF THOSE OF 

THE SAME WEIGHT AS WATER.-OF THOSE LIGHTER TUAN WATER.-OF 

THE SPECIFIC GRAVITY OF FLUIDS. 

MRS. B. 

k We have hitherto confined our attention to the mechani¬ 
cal properties of solid bodies, which have been illustrated, 
and, I hope, thoroughly impressed upon your memory, by the 
conversations we have subsequently had on astronomy. It 
will now be necessary for me to give you some account of 
the mechanical properties of fluids—a science which is call¬ 
ed hydrostatics. A fluid is a substance which yields to the 
slightest pressure. If you dip your hand into a basin of 
water, you are scarcely sensible of meetiug with any resist¬ 
ance. 

Emily. The attraction of cohesion is then, I suppose, 
less powerful in fluids than in solids? 

Mrs. B. Yes; fluids, generally speaking, are bodies of 
less density than solids. From the slight cohesion, of the 
particles of fluids, and the facility with which they slide 
over each other, it is inferred, that they must be small, smooth, 
and globular; smooth, because there appears to be little or 


MECHANICAL PROPERTIES OF FLUIDS. 133 

no friction among them; and globular, because touching each 
other but by a point would account for the slightness of their 
cohesion 

Caroline. Pray what is the distinction between a fluid 
and a liquid? 

Mrs. B. Liquids comprehend only one class of fluids. 
There is another class distinguished by the name of elastic 
fluids, or gases, which comprehends the air of the atmos¬ 
phere, and all the various kinds of air with which you will 
become acquainted when you study chemistry. Their me¬ 
chanical properties we shall examine at our next meeting, 
and confine our attention this morning to those of liquids, or 
non-elastic fluids. 

Water, and liquids in general, are scarcely susceptible of 
being compressed, or squeezed into a smaller space than 
that which they naturally occupy. This is supposed to be 
owing to the extreme minuteness of their particles, which, 
rather than submit to compression, force their way through 
the pores of the substance which confines them. This was 
shown by a celebrated experiment, made at Florence many 
years ago. A hollow globe of gold was filled with water, 
and on its being submitted to great pressure, the water was 
seen to exude through the pores of the gold, which it cover¬ 
ed with a fine dew. Fluids gravitate in a more perfect 
manner than solid bodies; for the strong cohesive attraction 
of the particles of the latter in some measure counteracts the 
effect of gravity. In this table, for instance, the cohesion 
of the particles of wood enables four slender legs to support 
a considerable weight. Were the cohesion destroyed, or, 
in other words, the wood converted into a fluid, no support 
could be afforded by the legs, for the particles no longer 
cohering together, each would press separately and inde¬ 
pendently, and would be brought to a level with the surface 
of the earth. 

Emily. This want of cohesion is then the reason why 
fluids can never be formed into figures, or maintained in 
heaps; for though it is true the wind raises water into waves, 
they are immediately afterwards destroyed by gravity, and 
water always finds its level. 

12* m £ 


134 MECHANICAL PROPERTIES OF FLUIDS. 

Mrs. B, Do you understand what is meant by the level, 
or equilibrium of fluids? 

Emily. I believe 1 do, though I feel rather at a loss to 
explain it. Is not a fluid level when its surface is smooth 
and flat, as is the case with all fluids when in a state of rest? 

Mrs. B. Smooth, if you please, but not flat; for the defi¬ 
nition of the equilibrium of a fluid is, that every part of the 
surface is equally distant from the point to which gravity 
tends, that is to say, from the centre of the earth; hence the 
surface of all fluids must be bulging, not flat, since they will 
partake of the spherical form of the globe. This is very 
evident in large bodies of water, such as the ocean, but the 
spericity of small bodies of water is so trifling, that their 
surfaces appear flat. 

This level, or equilibrium of fluids is the natural result of 
their particles gravitating independently of each other; for 
for when any particle of a fluid accidentally finds itself tie-. 
vated above the rest, it is attracted down to the level of the 
surface of the fluid, and the readiness with which fluids yield 
to the slightest impression, will enable the particle by its 
weight to penetrate the surface of the fluid and mix with it. 

C aroline. But I have seen a drop of oil float on the sur¬ 
face of water without mixing with it. 

Mrs. B. That is, because oil is a lighter liquid than 
water. If you were to pour water over it, the oil would 
rise to the surface, being forced up by the superior gravity of 
the water. Here is an instrument called a water-level (fig. 
1. plate XIII.) which is constructed upon the principle of 
the equilibrium of fluids. It consists of a short tube A B, 
closed at both ends, and containing a little water; when the 
tub is not perfectly horizontal the water runs to the lower 
end, and it is by this means that the level of any situation, 
to which we apply the instrument, is ascertained. 

Solid bodies you may, therefore, consider as gravitating 
in mrisses, for the strong cohesion of their particles makes 
them weigh altogether, while every particle of a fluid may be 
considered as composing a separate mass, gravitating inde¬ 
pendently of each other. Hence the resistance of a fluid is 
considerably less than that of a solid body; for the resistance 


1*1 .ATK XIII 



X 

















































































































































• ; (, 








* / ,* 










>. < s 













• 










• l 










MECHANICAL PROPERTIES OF FLUIDS. 135 

of the particles acting separately, they are more easily over¬ 
come. 

Emily . A body of water, in falling, does certainly less 
injury than a solid body of the same weight. 

Mrs . B. The particles of fluids acting thus independent¬ 
ly, press against each other in every direction, not only down¬ 
wards but upwards, and laterally or sideways; and in conse¬ 
quence of this equality of pressure, every particle remains at 
rest in the fluid. If you agitate the fluid you disturb this 
equality of pressure, and the fluid will not rest till its equili¬ 
brium is restored. 

Caroline. The pressure downwards is very natural; it is 
the effect of gravity, one particle weighing upon another 
presses on it; but the pressure sideways, and particularly 
the pressure upwards, I can not understand. 

Mrs. B. If there were no lateral pressure, water would 
not run out of an opening on the side of a vessel. If you 
'’I a vessel with sand, it will not run out of such an open¬ 
ing, because there is scarcely any lateral pressure among its 
particles. 

Emily. When w ater runs out of the side of a vessel, is 
it not owing to the weight of the water above the opening? 

Mrs. B. If the particles of fluids were arranged in regu¬ 
lar columns thus, (fig. 2.) there would be no lateral pressure, 
for when one particle is perpendicularly above the other, it 
can only press downwards; but as it must continually hap¬ 
pen that a particle presses between two particles beneath, 
(fig. 3.) these last must suffer a lateral pressure. 

Emily. The same as when a wedge is driven into a 
piece of wood, and separates the parts laterally. 

Mrs. B. Yes. The lateral pressure proceeds, therefore, 
entirely from the pressure downwards, or the weight of the 
liquid above; and consequently the lower the orifice is made 
in the vessel, the greater will be the velocity of the water 
rushing out of it. Here is a vessel of water (fig. 5.), with 
three stop cocks at different heights; we shall open them, 
and you will see with what different degrees of velocity the 
water issues from them. Do you understand this, Caroline? 

Caroline . Oh yes. The water from the upper spout re- 


136 MECHANICAL PROPERTIES OP FLUIDS. 

ceiving but a slight pressure, on account of its vicinity to 
the surface, flows but gently; the second cock having a 
greater weight above it, the water is forced out with greater 
velocity, whilst the lowest cock being near the bottom of 
the vessel receives the pressure of almost the whole body of 
water, and rushes out with the greatest impetuosity. 

Mrs. B. Very well; and you must observe, that as the 
lateral pressure is entirely owing to the pressure downwards, 
it is not effected by the horizontal dimensions of the vessel, 
which contains the water, but merely by its depth; for as 
every particle acts independently of the rest, it is only the 
column of particles, immediately above the orifice, that can 
wejgh upon and press out the water. 

Emily. The breadth and width of the vessel then can 
be of no consequence in this respect. The lateral pressure 
on one side, in a cubical vessel, is, I suppose, not so great 
as the pressure downwards. 

Mrs. B. No; in a cubical vessel the pressure downwards 
will be double the lateral pressure on one side; for every 
particle at the bottom of the vessel is pressed upon by a co¬ 
lumn of the whole depth of the fluid, whilst the lateral pres¬ 
sure diminishes from the bottom upwards to the surface, 
where the particles have no pressure. 

Caroline. And from whence proceeds the pressure of 
fluids upwards? that seems to me the most unaccountable, 
as it is in direct opposition to gravity. 

Mrs. B. And yet it is in consequence of their pressure 
downwards. When, for example, you pour water into a tea¬ 
pot, the water rises in the spout to a level with the water in 
the pot. The particles of water at the bottom of the pot 
are pressed upon by the particles above them; to this pres¬ 
sure they will yield, if there is any mode of making way for 
the superior particles, and as they can not descend, they will 
change their direction and rise in the spout. 

Suppose the tea-pot to be filled with columns of particles 
of water similar to that described in fig. 4., the particle 1 
at the bottom will be pressed laterally by the particle 2 , 
and by this pressure be forced into the spout, where meet¬ 
ing with the particle 3, it presses it upwards, and this pres- 


MECHANICAL PROPERTIES OP FLUIDS. 137 

sure will be continued from 3 to 4, from 4 to 5, and so on 
till the water in the spout has risen to a level with that in 
the pot. 

Emily. If it were not for this pressure upwards, forcing 
the water to rise in the spout, the equilibrium of the fluid 
would be destroyed. 

Caroline. True; but then a tea-pot is wide and large, 
and the weight of so great a body of water as the pot will 
contain, may easily lorce up and support so small a quantity 
as will fill the spout. But would the same effect be pro¬ 
duced if the spout and the pot were of equal dimensions? 

Mrs. B. Undoubtedly it would. You may even reverse 
the experiment by pouring water into the spout, and you 
will find that the water will rise in the pot to a level with 
that in the spout; for the pressure of the small quantity of 
water in the spout will force up and support the larger quan¬ 
tity in the pot. In the pressure upwards, as well as that 
laterally, you see that the force of pressure depends entirely 
on the height, and is quite independent of the horizontal 
dimensions of the fluid. 

As a tea-pot is not transparent, let us try the experiment 
by filling this large glass goblet by means of this narrow tube, 
(fig. 6.) 

Caroline . Look, Emily, as Mrs. B. fills it, how the wa¬ 
ter rises in the goblet, to maintain an equilibrium with that 
in the tube. 

Now, Mrs. B , will you let me fill the tube by pouring 
water into the goblet? 

Mrs. B. That is impossible. However, you may try 
the experiment, and I doubt not but that you will be able to 
account for its failure. 

Caroline. It is very singular, that if so small a column 
of water as is contained in the tube can force up and sup¬ 
port the whole contents of the goblet; that the weight of all 
the water in the. goblet should not be able to force up the 
small quantity required to fill the tube:—oh, I see now the 
reason, the water in the goblet can not force that in the tubs 
above its level, and as the end of the tube is considerably 


138 MECHANICAL PROPERTIES OP FLUIDS. 

higher than the goblet, it can never be filled by pouring wa¬ 
ter into the goblet. 

Mrs. B And if you continue to pour water into the gob¬ 
let when it is full, the water will run over instead of rising 
above the level in the tube. 

I shall now explain to you the meaning of the specific 
gravity of bodies. 

Caroline. What! is there another species of gravity with 
which we are not yet acquainted? 

Mrs. B. No: the specific gravity of a body, means sim- * 
ply its weight compared with that of another body of the 
same size. When we say, that substances such as lead and 
stones are heavy, and that others, such as paper and fea¬ 
thers, are light, we speak comparatively; that is to say, that 
the first are heavy, and the latter light, in comparison with 
the generality of substances in nature. Would you call 
wood and chalk light or heavy bodies? 

Caroline. Some kinds of wood are heavy certainly, as 
oak and mahogany; others are light, as deal and box. 

Emily. I think I should call wood in general a heavy 
body, for deal and box are light only in comparison to wood 
of a heavier description. I am at a loss to determine whe¬ 
ther chalk should be ranked as a heavy or a light body; I 
should be inclined to say the former, if it was not that it is 
lighter than most other minerals. I perceive that we have 
but vague notions of light and heavy. I wish there was 
some standard of comparison, to which we could refer the 
weight of all other bodies. 

Mrs. B. The necessity of such a standard has been so 
much felt, that a body has been fixed upon for this purpose. 
What substance do you think would be best calculated to 
answer this end? 

Caroline . It must be one generally known and easily 
obtained, lead or iron, for instance. 

Mrs. B. All the metals expand by heat, and condense 
by cold. A piece of lead, let us say a Gubic inch, for in¬ 
stance, would have less specific gravity in summer than in 
winter; for it would be more dense in the latter season. 


MECHANICAL PROPERTIES OF FLUIDS. 139 

Caroline . But, Mrs, B., if you compare the weight of 
equal quantities of different bodies, they will all be alike. 
You know the old saying, that a pound of feathers is as heavy 
as a pound of lead? 

Mrs. B. When therefore we compare the weight of 
different kinds of bodies, it would be absurd to take quanti¬ 
ties of equal weight , we must take quantities of equal bulk; 
pints or quarts, not ounces or pounds. 

Caroline. Very true; I perplexed myself by thinking that 
quantity referred to weight, rather than to measure It is 
true, it would be as absurd to compare bodies of the same 
size in order to ascertain which was largest, as to compare 
bodies of the same weight in order to discover which was 
heaviest. 

Mrs. B. In estimating the specific gravity of bodies, 
therefore, we must compare equal bulks, and we shall find 
that their specific gravity will be proportional to their 
weights. The body which has been adopted as a standard 
of reference is distilled water. 

Emily . I am surprised that a fluid should have been 
chosen for this purpose, as it must necessarily be contained 
in some vessel, and the weight of the vessel will require to 
be deducted. 

Mrs. B. In order to learn the specific gravity of a solid 
body, it is not necessary to put a certain measure of it in 
one scale, and an equal measure of water into the other 
scale: but simply to weigh the body under trial in water. 
If you weigh a piece of gold in a glass of water will not the 
gold displace just as much water as is equal to its own bulk? 

Caroline. Certainly, where oue body is, another can not 
be at the same time; so that a sufficient quantity of water 
must be removed, in order to make way for the gold. 

Mrs. B. Yes, a cubic inch of water to make room for a 
cubic inch of gold; remember that the bulk alone is to be 
considered, the weight has nothing to do with the quantity 
of water displaced, for an inch of gold does not occupy more 
space, and therefore will not displace more water than an 
inch of ivory, or any other substance, that will sink in water. 


140 


MECHANICAL PROPERTIES OF FLUIDS. 


Well, you will perhaps be surprised to hear that the gold 
will weigh less in water,, than it did out of it? 

Emily. And for what reason? 

Mrs. B. On account of the upward pressure of the par¬ 
ticles of water, which in some measure supports the gold, 
and by so doing, diminishes its weight. li the body im¬ 
mersed in water was of the same weight as that fluid, it 
would be wholly supported by it, just as the water which it 
displaces was supported previous to its making way for the 
solid body. If the body is heavier than the water, it can not 
be wholly supported by it; but the water will offer some re¬ 
sistance to its descent. 

Caroline. And the resistance which water offers to the 
descent of heavy bodies immersed in it, (since it proceeds 
from the upward pressure of the particles of the fluid), must 
in all cases, I suppose, be the same? 

Mrs. B. Yes: the resistance of the fluid is proportioned 
to the bulk, and not to the weight of the body immersed in 
it; all bodies of the same size, therefore, lose the same 
quantity of their weight in water. Can you form any idea 
what this loss will be. 

Emily. I should think it would be equal to the weight 
of the water displaced; for, since that portion of the water 
was supported before the immersion of the solid body, an 
equal weight of the solid body will be supported. 

Mrs. B. You are perfectly right; a body weighed in wa¬ 
ter loses just as much of its weight, as is equal to that of 
the water it displaces; so that if you were to put the water 
displaced into the scale to which the body is suspended, it 
would restore the balance. 

You must observe, that when you weigh a body in water, 
in order to ascertain its specific gravity, you must not sink 
the bason of the balance in the water; but either suspend 
the body to a hook at the bottom of the bason, or else take 
off the bason, and suspend it to the arm of the balance, (fig. 
7.) Now suppose that a cubic inch of gold weighed 19 
ounces out of water, and lost one ounce of its weight by be¬ 
ing weighed in water, what would be its specific gravity? 

Caroline. The cubic inch of water it displaced must 


MECHANICAL PROPERTIES OP FLUIDS. 141 

weigh that one ounce; and as a cubic inch of gold weighs 
19 ounces, gold is 19 times as heavy as water. 

Emily. I recollect having seen a table of the compara¬ 
tive weights of bodies, in which gold appeared to me to be 
estimated at 19 thousand times the weight of water. 

Mrs. B. You misunderstood the meaning of the table. 
In the estimation you allude to, the weight of water was 
reckoned at 1000. You must observe, that the weight of 
a substance when not compared to that of any other, is per¬ 
fectly arbitrary; and when water is adopted as a standard, 
we may denominate its weight by any number we please; 
but then the weight of all bodies tried by this standard must 
be signified by proportional numbers. 

Caroline. We may call the weight of water, for exam¬ 
ple, one, and then that of gold would be nineteen; or if we 
choose to call the weight of water 1000, that of gold would be 
19,000. In short, the specific gravity means how much 
more a body weighs than an equal bulk of water. 

Mrs. B. It is rather the weight of a body compared with 
that of water; for the specific gravity of many substances is 
less than that of water. 

Caroline. Then you can not ascertain the specific gra¬ 
vity of such substances in the same manner as that of gold; 
for a body that is lighter than water will float on its surface 
without displacing any water. 

Mrs. B. If a body were absolutely light, it is true that 
it would not displace a drop of water, but the bodies we are 
treating of have all some weight, however small; and will 
therefore, displace some quantity of water. If the body be 
lighter than water, it will not sink to a level with the sur¬ 
face of the water, and therefore it will not displace so much 
water as is equal to its bulk; but it will displace as much as 
is equal to its weight. A ship, you must have observed, 
sinks to some depth in water, and the heavier it is laden the 
deeper it sinks, as it always displaces a quantity of water 
equal to its weight. 

Caroline. But you said just now, that in the immersion 
of gold, the bulk, and not the weight of body, was to be 
considered. 


13 





142 MECHANICAL PROPERTIES OF FLUIDS. 

Mrs. B. That is the case with all substance which are 
Sieavier than water; but since those which are lighter do 
not displace so much as their own bulk, the quantity they 
displace is not a test of their specific gravity. 

In order to obtain the specific gravity of a body which is 
lighter than water, you must attach to it a heavy one, whose 
specific gravity is known, and immerse them together; the 
specific gravity of the lighter body may then be easily cal¬ 
culated. 

Emily. But are there not some bodies which have ex¬ 
actly the same specific gravity as w r ater? 

Mrs. B. Undoubtedly; and such bodies will remain at 
rest in whatever situation they are placed in water. Here 
is a piece of wood which by being impregnated with a little 
sand, is rendered precisely of the weight of an equal bulk of 
water; in whatever part of this vessel of water you place it, 
you will find that it will remain stationary. 

Caroline. I shall first put it at the bottom; from thence, 
of course, it can not rise, because it is not lighter than wa¬ 
ter. Now I shall place it in the middle of the vessel; it nei¬ 
ther rises nor sinks, because it is neither lighter nor heavier 
than the water. Now I will lay it on the surface of the wa¬ 
ter; but there it sinks a little—what is the reason of that, 
Mrs. B: ? 

Mrs B. Since it is not lighter than the water, it can 
not float upon its surface; since it is not heavier than water, 
it can not sink below its surface: it will sink therefore, only 
till the upper surface of both bodies are on a level, so that 
the piece of wood is just covered with water. If you pour¬ 
ed a few drops of water into the vessel, (so gently as not to 
increase their momentum by giving them velocity) they 
would mix with the water at the surface, and not sink lower. 

Caroline. This must, no doubt, be the reason why in 
drawing up a bucket of water out of a well, the bucket feels 
so much heavier when it rises above the surface of the wa¬ 
ter in the well; for whilst you raise it in the water, the water 
within the bucket being of the same specific gravity as the 
water on the outside, will be wholly supported by the up¬ 
ward pressure of the water beneath the bucket, and conse- 


MECHANICAL PROPERTIES OF FLUIDS. 143 

quently very little force will be required to raise it; but as 
soon as the bucket rises to the surface of the well, you im¬ 
mediately perceive the increase of weight. 

Emilu. And how do you ascertain the specific gravitv 
of fluids? 

J\lrs. B. By means of an instrument called an hydro¬ 
meter, which I will show you. It consists of a thin glass 
ball A, (fig. 8, plate XIII.) with a graduated tube B, and 
the specific gravity of the liquid is estimated by the depth 
to which the instrument sinks in it. There is a smaller ball, 
C, attached to the instrument below which contains a little 
mercury; but this is merely for the purpose of equipoising 
the instrument, that it may remain upright in the liquid un¬ 
der trial. 

I must now take leave of you; but there remain yet many 
observations to be made on fluids: we shall, therefore re- 
same this subject at our next interview. 


CONVERSATION XI. 


OF SPRINGS, FOUNTAINS, &c. 


OF THE ASCENT OF VAPOUR AND THE FORMATION OF CLOUDS.-OF THE FOR¬ 

MATION AND FALL OF RAIN, &C.—OF THE FORMATION OF SPRINGS.—OF 

RIVERS AND LAKES.—OF FOUNTAINS. 

CAROLINE. 

There is a question I am very desirous of asking you re¬ 
respecting fluids, Mrs. B., which has often perplexed me. 
What is the reason that the great quantity of rain which falls 
upon the earth and sinks into it, does not, in the course of 
time, injure its solidity? The sun and the wind, I know, dry 
the surface, but they have no effect on the interior parts, 
where there must be a prodigious accumulation of moisture. 

Mrs. B. Do you not know that, in the course of time, 
all the water which sinks into the ground rises out of it again? 
It is the same water which successively forms seas, rivers, 
springs, clouds, rain, and sometimes hail, snow and ice. If 
you will take the trouble of following it through these various 
changes, you will understand why the earth is not yet 
drowned by the quantity of water which has fallen upon it 
since its creation; and you will even be convinced, that it 
does not contain a single drop more water now, than it did 
at that period. 

Let us consider how the clouds were originally formed. 
When the first rays of the sun warmed the surface of the 
earth, the heat, by separating the particles of water, render- 


ON SPRINGS, FOUNTAINS, &C. 145 

ed them lighter than the air. This, you know, is the case 
with steam or vapour. What then ensues? 

Caroline. When lighter than the air it will naturally 
rise; and now I recollect your telling us in a preceding les¬ 
son, that the heat of the sun transformed the particles of wa¬ 
ter into vapour, in consequence of which it ascended into the 
atmosphere, where it formed clouds. 

Mrs. B. We have then already followed water through 
two of its transformations; from water it becomes vapour, 
and from vapour clouds. 

Emily. But since this watery vapour is lighter than the 
air, why does it not continue to rise; and why does it unite 
again to form clouds? 

Mrs. B. Because the atmosphere diminishes in density, 
as it is more distant from the earth. The vapour, therefore, 
which the sun causes to exhale, not only from seas, rivers, 
and lakes, but likewise from the moisture on the land, rises 
till it reaches a region of air of its ow T n specific gravity; and 
there, you know, it will remain stationary. By the frequent 
accession of fresh vapour it gradually accumulates, so as to 
form those large bodies of vapour, which we call clouds: 
and these, at length, becoming too heavy for the air to support, 
they fall to the ground. 

Caroline . They do fall to the ground, certainly, when 
it rains; but, according to your theory, 1 should have im¬ 
agined, that when the clouds became too heavy for the region 
of air in which they were situated to support them, they 
would descend till they reached a stratum of air of their own 
weight, and not fall to the earth; for as clouds are formed of 
vapour, they can not be so heavy as the lowest regions of 
the atmosphere, otherwise the vapour tvould not have risen. 

Mrs. B. If you examine the manner in which the clouds 
descend, it will obviate this objection. In falling, several 
of the watery particles come within the sphere of each 
other’s attraction, and unite in the form of a drop of water. 
The vapour thus transformed into a shower, is heavier than 
any part of the atmosphere, and consequently descends to 
the earth. 

Caroline . How wonderfully curious! 

13 * 


146 ON SPRINGS, FOUNTAINS, &C. 

Mrs. B. It is impossible to consider any part of nature 
attentively without being struck with admiration at the wis¬ 
dom it displays; and I hope you will never contemplate these 
wonders without feeling your heart glow with admiration 
and gratitude towards their bounteous Author. Observe, 
that if the waters were never drawn out of the earth, all 
vegetation would be destroyed by the excess of moisture; if, 
on the other hand, the plants were not nourished and re¬ 
freshed by occasional showers, the drought would be equally 
fatal k to them. If the clouds constantly remained in a 
state of vapour, they might, as you remarked, descend into 
a heavier stratum of the atmosphere, but could never fall to 
the ground; or were the power of attraction more than suf¬ 
ficient to convert the vapour into drops, it would transform 
the cloud into a mass of water, which, instead of nourishing 
would destroy the produce of the earth. 

Water then ascends in the form of vapour, and descends 
in that of rain, snow, or hail, all of which ultimately become 
water. Some of this falls into the various bodies of water 
on the surface of the globe, the remainder upon the land. 
Of the latter, part reascends in the form of vapour, part is 
absorbed by the roots of vegetables, and part descends into 
the bowels of the earth, where it forms springs. 

Emily. Is rain and spring-water then the same? 

Mrs. B. Yes, originally. The only difference between 
rain ami spring water, consists in the foreign particles which 
the latter meets with and dissolves in its passage through 
the various soil it traverses. 

Caroline. Yet spring-water is more pleasant to the taste, 
appears more transparent, and, I should have supposed, 
would have been more pure than rain water. 

Mrs. B. No; excepting distilled water, rain water is the 
most pure we can obtain; and it is its purity which renders 
it insipid, whilst the various salts and different ingredients, 
dissolved in spring water, give it a species of flavour, with¬ 
out in any degree affecting its transparency, and the filtra¬ 
tion it undergoes through gravel and sand in the bowels of 
the earth, cleanses it from all foreign matter which it has 
not the power of dissolving. 


ON SPRINGS, FOUNTAINS, &C. 141 

When rain falls on the surface of the earth, it continues 
making its way downwards through the pores and crevices 
in the ground. When several drops meet in their subter¬ 
raneous passage, they unite and form a little rivulet; this, in 
its progress, meets with other rivulets of a similar descrip¬ 
tion, and they pursue their course together in the bowels of 
the earth, till they are stopped by some substance which they 
can not penetrate. 

Caroline. But you say that water could penetrate even 
the pores of gold, and they can not meet with a substance 
more dense? 

Mrs. B. But water penetrates the pores of gold only 
when under a strong compressive force, as in the Floren¬ 
tine experiment; now in its passage towards the centre of 
the earth, it is acted upon by no other power than gravity, 
which is not sufficient to make it force its way even through 
a stratum of clay. This species of earth, though not re¬ 
markably dense, being of great tenacity, will not admit the 
particles of water to pass. When water encounters any 
substance of this nature, therefore, its progress is stopped, 
and the pressure of the accumulating waters forms a bed, or 
reservoir. This will be more clearly explained by fig. 9. 
plate XIII. which represents a section, or the interior of a 
hill or mountain. A, is a body of water such as I have de¬ 
scribed, which, when filled up as high asB, (by the continual 
accession of water it receives from the ducts or rivulets a , 
a, a,) finds a passage out of the cavity, and, impelled by 
gravity, it runs on, till it makes its way out of the. ground at 
the side of the hill, and there forms a spring, C. 

Caroline. Gravity impels downwards towards the centre 
of the earth; and the spring in this figure runs in an hori¬ 
zontal direction. 

Mrs . B. Not entirely. There is some declivity from 
the reservoir to the spot where the water issues out of the 
ground; and gravity you know will bring bodies down an 
inclined plane, as well as in a perpendicular direction. 

Caroline . But though the spring may descend, on first 
issuing, it must afterwards rise to reach the surface of the 
earth: and that is in direct opposition to gravity. 


148 


ON SPRINGS, FOUNTAINS, &C, 

Mrs. B. A spring can never rise above the level of the 
reservoir whence it issues; it must, therefore, find a passage 
to some part of the surface of the earth that is lower or near¬ 
er the centre than the reservoir. It is true that, in this 
figure, the spring rises in its passage from B to C occasion¬ 
ally; but this, I think, with a little reflection, you will be 
able to account for. 

Emily. Oh, yes; it is owing to the pressure of fluids up¬ 
wards, and the water rises in the duct upon the same prin¬ 
ciple as it rises in the spout of a tea-pot; that is to say, in 
order to preserve an equilibrium with the water in the re¬ 
servoir. Now I think I understand the nature of springs: the 
water will flow through a duct, whether ascending or de¬ 
scending, provided it never rises higher than the reservoir. 

Mrs. B. Water may thus be conveyed to every part of 
a town, and to the upper part of the houses, if it is originally 
brought from a height superior to any to which it is con¬ 
veyed. Have you never observed, when the pavements of 
the streets have been mending, the pipes which serve as 
ducts for the conveyance of the water through the town? 

Emily. Yes, frequently; and I have remarked that when 
any of these pipes have been opened, the water rushes up¬ 
wards from them with great velocity, which, I suppose, pro¬ 
ceeds from the pressure of the water in the reservoir, which 
forces it out. 

Caroline. I recollect having once seen a very curious 
glass, called Tantalus’s cup; it consists of a goblet, con¬ 
taining a small figure of a man, and whatever quantity of 
water you pour into the goblet, it never rises higher than the 
breast of the figure. Do you know how that is contrived? 

Mrs. B. It is by means of a syphon, or bent tube, which 
is concealed in the body of the figure. It rises through one 
of the legs as high as the breast, and there turning, descends 
through the other leg, and from thence through the Toot of the 
goblet, where the water runs out. (fig. 1. plate XIY.) When 
you pour water into the glass A, it must rise in the syphon 
B, in proportion as it rises in the glass; and when the glass 
is filled to a level with the upper part of the syphon, the 
water will run out through the other leg of the figure, and 


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ON SPRINGS, FOUNTAINS, &C. 149 

will continue running out, as fast as you pour it in; therefore 
the glass can never fill any higher. 

Emily . I think the new well that has been made at our 
country-house, must be of that nature. We had a great 
scarcity of water, and my father has been at considerable 
expense to dig a Veil; after penetrating to a great depth 
before water could be found, a spring was at length disco¬ 
vered, but the water rose only a few feet above the bottom 
of the well; and sometimes it is quite dry. 

Mrs. B. This has, however, no analogy to Tantalus’s 
cup; but is owing to the very elevated situation of your 
country-house. 

Emily. I believe I guess the reason. There can not be 
a reservoir of water near the summit of a hill; as in such a 
situation, there will not be a sufficient number of rivulets 
formed to supply one; and without a reservoir, there can be 
no spring. In such situations, therefore, it is necessary to 
dig very deep, in order to meet with a spring; and when we 
give it vent, it can rise only as high as the reservoir from 
whence it flows, which will be but little, as the reservoir 
must be situated at some considerable depth below the sum¬ 
mit of the hill. 

Caroline. Your explanation appears very clear and 
satisfactory; but I can contradict it from experience. At 
the very top of a hill, near our country-house, there is a 
large pond, and according to your theory, it would be im¬ 
possible there should be springs in such a situation to supply 
it with water. Then you know that I have crossed the 
Alps, and I can assure you, that there is a fine lake on the 
summit of Mount Cenis, the highest mountain we passed 
over. 

Mrs. B. Were there a lake on the summit of Mount 
Blanc, which is the highest of the Alps, it would indeed be 
wonderful. But that on Mount Cenis, is not at all contra¬ 
dictory to our theory of springs; for this mountain is sur¬ 
rounded by others, much more elevated, and the springs 
which feed the lake must descend from reservoirs of water 
formed in those mountains. This must also be the case 
with the pond on the top of the hill; there is doubtless some 


150 ON SPRINGS, FOUNTAINS, &C. 

more considerable hill in the neighbourhood, which supplies 
it with water. 

Emily. I comprehend perfectly, why the water in our 
well never rises high: but I do not understand why it should 
occasionally be dry. 

Mrs. B. Because the reservoir from which it flows, be¬ 
ing in an elevated situation, is but scantily supplied with 
w r ater; after a long drought, therefore, it may be drained, 
and the spring dry, till the reservoir be replenished by fresh 
rains. It is not uncommon to see springs flow with great 
violence in w T et weather, and at other times be perfectly 
dry. 

Caroline. But there is a spring in our grounds which 
more frequently flows in dry than in w r et weather; how is 
that to be accounted for? 

Mrs. B. The spring probably comes from a reservoir 
at a great distance, and situated very deep in the ground: it 
is, therefore, some length of time before the rain reaches 
the reservoir, and another considerable portion must elapse, 
whilst the water is making its way, from the reservoir to 
the surface of the earth; so that the dry weather may proba¬ 
bly have succeeded the rains before the spring begins to flow, 
and the reservoir may be exhausted by the time the wet 
weather sets in again. 

Caroline. I doubt not but this is the case, as the spring 
is in a very low situation, therefore the reservoir may be at 
a great distance from it. 

Mrs. B. Springs which do not constantly flow, are called 
intermitting, and are occasioned by the reservoir being im¬ 
perfectly supplied. Independently of the situation, this is 
always the case when the duct or ducts which convey the 
water into the reservoir are smaller than those which carry 
it off. 

Caroline. If it runs out faster than it runs in, it will of 
course sometimes be empty. And do not rivers also derive 
their source from springs? 

Mrs. B. Yes, they generally take their source in moun¬ 
tainous countries where springs are most abundant. 


ON SPRINGS, FOUNTAINS, &C. 151 

Caroline. I understood you that springs were more rare 
in elevated situations. 

Mrs. B. You do not consider that mountainous coun¬ 
tries abound equally with high and low situations. Reser¬ 
voirs of water, which are formed in the bosoms of moun¬ 
tains, generally find a vent either on their declivity, or in 
the valley beneath; while subterraneous reservoirs formed in 
a plain, can seldom find a passage to the surface of the earth, 
but remain concealed, unless discovered by digging a well. 
When a spring once issues at the surface of the earth it 
continues its course externally, seeking always a lower 
ground, for it can no longer rise. 

Emily. Then what is the consequence, if the spring, or 
I should now rather call it a rivulet, runs into a situation, 
which is surrounded by higher ground? 

Mrs. B. Its course is stopped, the water accumulates, 
and it forms a pool, pond, or lake, according to the dimen¬ 
sions of the body of water. The lake of Geneva, in all pro¬ 
bability, owes its origin to the Rhone, which passes through 
it: if, when this river first entered the valley, which now 
forms the bed of the Lake, it found itself surrounded by 
higher grounds, its waters would there accumulate, till they 
rose to a level with that part of the valley, where the Rhone 
now continues its course beyond the Lake, and from whence 
it flows through valleys, occasionally forming other small 
lakes till it reaches the sea. 

Emily. And are not fountains of the nature of springs? 

Mrs. B. Exactly. A fountain is conducted perpendi¬ 
cularly upwards, by the spout or adjutage A, through which 
it flows; and it will rise nearly as high as the reservoir B, 
from whence it proceeds. (Plate XIV. fig. 2.) 

Caroline. Why not quite as high? 

Mrs. B. Because it meets with resistance from the air 
in its ascent; and its motion is impeded by friction against 
the spout, where it rushes out. 

Emily. But if the tube through which the water rises be 
smooth, can there be any friction? especially with a fluid, 
whose particles yield to the slightest impression. 


15 % ON SPRINGS, FOUNTAINS, &C. 

Mrs . B . Friction, (as we observed in a former lesson,) 
may be diminished by polishing, but can never be entirely 
destroyed; and though fluids are less susceptible of friction 
than solid bodies, they are still affected by it. Another rea¬ 
son why a fountain will not rise so high as its reservoir, is, 
that as all the particles of water spout from the tube with 
an equal velocity, and as the pressure of the air upon the 
exterior particles must diminish their velocity, they will in 
some degree strike against the under parts, and force them 
sideways, spreading the column into a head, and rendering 
it both wider and shorter than it otherwise would be. 

At our next meeting, we shall examine the mechanical 
properties of the air, which being an elastic fluid, differs in 
many respects from liquids. 


CONVERSATION XII. 


ON THE MECHANICAL PROPERTIES OP AIR. 


OP THE SPRING OR ELASTICITY OF THE AIR.—OF THE WEIGHT OF THE AIR, 

-EXPERIMENTS WITH THE AIR PUMP--OF THE BAROMETER.-MODE OP 

WEIGHING AIR.-SPECIFIC GRAVITY OF AIR.-OF PUMPS.—DESCRIPTION 

OF THE SUCKING PUMP.-DESCRIPTION OF THE FORCING PUMP. 

MRS. B. 

At our last meeting we examined the properties of fluids 
in general, and more particularly of such fluids as are called 
liquids. 

There is another class of fluids, distinguished by the name 
of aeriform or elastic fluids, the principal of which is the 
air we breathe, which surrounds the earth, and is called the 
atmosphere. 

Emily. There are then other kinds of air besides the 
atmosphere? 

Mrs. B. Yes; a great variety; but they differ only in 
their chemical, and not in their mechanical properties; and 
as it is the latter we are to examine, we shall not at present 
inquire into their composition, but confine our attention to 
the mechanical properties of elastic fluids in general. 

Caroline. And from whence arises this difference? 

Mrs. B. There is no attraction of cohesion between the 
particles of elastic fluids; so that the expansive power of heat 
has no adversary to contend with but gravity; any increase 
of temperature, therefore, expands elastic fluids prodigiously, 
and a diminution proportionally condenses them. 

14 


154 MECHANICAL PROPERTIES OF AIR. 

The most essential point in which air differs from other 
fluids, is by its spring or elasticity; that is to say, its power 
of increasing or diminishing in bulk, according as it is more 
or less compressed: a power of which I have informed you 
liquids are almost wholly deprived. 

Emily. I think I understand the elasticity of the air very 
well from what you formerly said of it; but what perplexes 
me is, its having gravity; if it is heavy, and we are surround¬ 
ed by it, why do we not feel its weight? 

Caroline. It must be impossible to be sensible of the 
weight of such infinitely small particles, as those of which 
the air is composed: particles which are too small to be seen, 
must be too light to be felt. 

Mrs. B. You are mistaken, my dear; the air is much 
heavier than you imagine; it is true, that the particles which 
compose it are small; but then, reflect on their quantity: the 
atmosphere extends to about the distance of 45 miles from 
the earth, and its gravity is such, that a man of middling 
stature is computed (when the air is heaviest) to sustain the 
weight of about 14 tons. 

Caroline. Is it possible! I should have thought such a 
weight would have crushed any one to atoms. 

Mrs. B. That would, indeed, be the case, if it were 
not for the equality of the pressure on every part of the body; 
but when thus diffused, we can bear even a much greater 
weight, without any considerable inconvenience. In bathing 
we support the weight and pressure of the water, in addi¬ 
tion to that of the atmosphere; but because this pressure is 
equally distributed over the body, we are scarcely sensible 
of it; whilst if your shoulders, your head, or any particular 
part of your frame were loaded with the additional weight 
of a hundred pounds, you would soon sink under the fatigue. 
Besides this our bodies contain air, the spring of which 
counterbalances the weight of the external air, and renders 
us less sensible of its pressure. 

Caroline. But if it were possible to relieve me from the 
weight of the atmosphere, should I not feel more light and 
agile? 

Mrs. B. On the contrary, the air within you meeting 


MECHANICAL PROPERTIES OP AIR. 


155 


with no external pressure to restrain its elasticity, would 
distend your body, and at length bursting the parts which 
confined it, put a period to your existence. 

Caroline. This weight of the atmosphere, then, which 
I was so apprehensive would crush me, is, in reality, essen¬ 
tial to my preservation. 

Emily , I once saw a person cupped, and was told that 
the swelling of the part under the cup was produced by 
taking away from that part the pressure of the atmosphere; 
but I could not understand how this pressure produced such 
an effect. 

* Mrs . B. The air pump affords us the means of making 
a great variety of interesting experiments on the weight and 
pressure of the air: some of them you have already seen. 
Do you not recollect, that in a vacuum produced within the 
air pump, substances of various weights fell to the bottom 
in tne same time; why does not this happen in the atmo¬ 
sphere? 

Caroline . I remember you told us it was owing to the 
resistance which light bodies meet with from the air during 
their fall. 

Mrs. B. Or, in other words, to the support which they 
received from the air, and which prolonged the time of their 
fall. Now, if the air were destitute of weight, how could it 
support other bodies or retard their fall? 

I shall now show you some other experiments, which 
illustrate, in a striking manner, both the weight and elasti¬ 
city of air. I shall tie a piece of bladder over this glass re¬ 
ceiver, which, you will observe, is open both at the top as 
well as below. 

Caroline. Why do you wet the bladder first? 

Mrs. B. It expands by wetting, and contracts in drying; 
it is also more soft and pliable when wet, so that I can make 
it fit better, and when dry it will be tighter. We must hold 
it to the fire in order to dry; but not too near lest it should 
burst by sudden contraction. Let us now fix it on the air- 
pump and exhaust the air from underneath it—you will not 
be alarmed if you hear a noise? 

Emily. It was as loud as the report of a gun, and the 


156 


MECHANICAL PROPERTIES OF AIR. 


bladder is burst! Pray explain how the air is concerned in 
this experiment. 

Mrs. B. It is the effect of the weight of the atmosphere 
on the upper surf ace of the bladder, when I had taken away 
the air from the under surface; so that there was no longer 
any reaction to counterbalance the pressure of the atmos¬ 
phere on the receiver. You observed how the bladder was 
pressed inwards by the weight of the external air, in propor¬ 
tion as I exhausted the receiver: and before a complete va¬ 
cuum was formed, the bladder, unable to sustain the vio¬ 
lence of the pressure, burst with the explosion you have just 
heard. 

I shall now show you an experiment, which proves the 
expansion of the air, contained within a body when it is 
relieved from the pressure of the external air. You would 
not imagine that there was any air contained within this 
shrivelled apple, by its appearance; but take notice of it 
when placed within a receiver, from which I shall exhaust 
the air. 

Caroline. How strange; it grows quite plump, and looks 
like a fresh-gathered apple. 

Mrs. B. But as soon as I let the air again into the re¬ 
ceiver, the apple you see returns to its shrivelled state. When 
I took away the pressure of the atmosphere the air within 
the apple expanded and swelled it out; but the instant the 
atmospherical air was restored, the expansion of the inter¬ 
nal air was checked and repressed, and the apple shrunk to 
its former dimensions. 

You may make a similar experiment with this little blad¬ 
der, which you see is perfectly flaccid, and appears to con¬ 
tain no air: in this state I shall tie up the neck of the blad¬ 
der, so that whatever air remains within it may not escape, 
and then place it under the receiver. Now observe, as I 
exhaust the receiver, how the bladder distends; this pro¬ 
ceeds from the great dilatation of the small quantity of air 
which was enclosed within the bladder when I tied it up; 
but as soon as I let the air into the receiver, that which the 
bladder contains, condenses and shrinks into its small com¬ 
pass within the folds of the bladder. 


MECHANICAL PROPERTIES OF AIR. 


157 


Emily. These experiments are extremely amusing, and 
they afford clear proofs both of the weight and elasticity of 
the air; but I should like to know exactly how much the air 
weighs. 

Mrs, B. A column of air reaching to the top of the at¬ 
mosphere, and whose base is a square inch, weighs 15!bs. 
when the air is heaviest; therefore every square inch of our 
bodies sustains a weight of 151bs.: and ifyou wish to know 
the weight of the whole of the atmosphere, you must reckon 
how many square inches there are on the surface of the 
globe, and multiply them by 15. 

Emily. But are there no means of ascertaining the 
weight of a small quantity of air? 

Mrs. B. Nothing more easy. I shall exhaust the air 
from this little bottle by means of the air-pump: and having 
emptied the bottle of air, or, in other words, produced a 
vacuum within it, I secure it by turning this screw adapted 
to its neck: we may now find the exact weight of this bottle, 
by putting it into one of the scales of a balance. It weighs 
you see just two ounces; but when 1 turn the screw so as to 
admit the air into the bottle, the scale which contains it pre¬ 
ponderates. 

Caroline. No doubt the bottle filled with air, is heavier 
than the bottle void of air; and the additional weight requir¬ 
ed to bring the scales again to a balance, must be exactly 
that of the air which the bottle now contains. 

Mrs. B. That weight, you see, is almost two grains. 
The dimensions of this bottle are six cubic inches. Six 
cubic inches of air, therefore, at the temperature of this 
room, weighs nearly 2 grains. 

Caroline. Why do you observe the temperature of the 
room, in estimating the weight of the air? 

Mrs. B. Because heat rarities air, and renders it lighter; 
therefore the warmer the air is which you weigh, the lighter 
it will be. 

Ifyou should now be desirous of knowing the specific 
gravity of this air, we need only fill the same bottle with 
water, and thus obtain the weight ot an equal quantity of 
water—which you see is 1515 grs.; now by comparing the 
14 * 


158 


MECHANICAL PROPERTIES OF AIR. 


weight of water to that of air, we find it to be in the pro¬ 
portion of about 800 to 1. 

I will show you another instance of the weight of the at¬ 
mosphere, which I think will please you: you know what a 
barometer is? 

Caroline. It is an instrument which indicates the state 
of the weather, by means of a tube of quicksilver; but how, 
I can not exactly say. , 

Mrs. B . It is by showing the weight of the atmosphere. 
The barometer is an instrument extremely simple in its 
construction: in order that you may understand it, I will 
show you how it is made. I first fill a glass tube A B, (fig. 
3. plate XlV.) about three feet in length, and open only at 
one end, with mercury;then stopping the open end with my 
finger, I immerse it in a cup C, containing a little mercury. 

Emily Part of the mercury which was in the tube, I 
observe, runs down into the cup; but why does not the whole 
of it subside in the cup, for it is contrary to the law of the 
equilibrium of fluids, that the mercury in the tube should 
not descend to a level with that in the cup? 

Mrs. B. The mercury that has fallen from the tube into 
the cup, has left a vacant space in the upper part of the 
tube, to which the air can not gain access; this space is 
therefore a perfect vacuum; and consequently the murcury 
in the tube is relieved from the pressure of the atmosphere, 
whilst that in the cup remains exposed to it. 

Caroline. Oh, now I understand it; the pressure of the 
air on the mercury in the cup forces it to rise in the tube, 
where it sustains no pressure. 

Emily. Or rather supports the mercury in the tube, and 
prevents it from falling. 

Mrs. B. That comes to the same thing; for the power 
that can support mercury in a vacuum, would also make it 
ascend when it met with a vacuum. 

Thus you see, that the equilibrium of the mercury is de¬ 
stroyed only to preserve the general equilibrium of fluids. 

Caroline. But this simple apparatus is, in appearance, 
very unlike a barometer. 

Mrs . B. It is all that is essential to a barometer. The 


MECHANICAL PROPERTIES OF AIR. 159 

tube and the cup or vase are fixed on a board, for the con¬ 
venience of suspending it; the board is graduated for the 
purpose of ascertaining the height at which the mercury 
stands in the tube; and the small moveable metal plate serves 
to snow that height with greater accuracy. 

Emily. And at what height will the weight of the at¬ 
mosphere sustain the mercury? 

Mrs. B. About 28 inches, as you will see by this baro¬ 
meter; but it depends upon the weight of the atmosphere, 
which varies much according to the state of the weather. 
The greater the pressure of the air on the mercury in the 
cup, the higher it will ascend in the tube. Now can you 
tell me whether the air is heavier in wet or in dry weather? 

Caroline. Without a moment’s reflection, the air must 
be heaviest in wet weather. It is so depressing, and makes 
one feel so heavy, while in fine weather, I feel as light as a 
feather, and as brisk as a bee. 

Mrs. B. Would it not have been better to have answer¬ 
ed with a moment’s reflection, Caroline? It would have 
convinced you, that the air must be heaviest in dry weather, 
for it is then, that the mercury is found to rise in the tube, 
and consequently the mercury in the cup must be most 
pressed by the air; and you know, that we estimate the dry¬ 
ness and fairness of the weather, by the height of the mer¬ 
cury in the barometer. 

Caroline. Why then does the air feel so heavy in bad 
weather? 

Mrs. B. Because it is less salubrious when impregnated 
with damp. The lungs under these circumstances do not 
play so freely, nor does the blood circulate so well; thus ob¬ 
structions are frequently occasioned in the smaller vessels, 
from which arises colds, asthmas, agues, fevers, fyc. 

Emily. Since the atmosphere diminishes in densily in 
the upper regions, is not the air more rare upon a hill than 
in a plain; and does the barometer indicate this difference? 

Mrs. B. Certainly. The hills in this country are not 
sufficiently elevated to produce any very considerable effect 
on the barometer; but this instrument is so exact in its in¬ 
dications, that it is used for the purpose of measuring the 


160 


MECHANICAL PROPERTIES OP AIR. 


height of mountains, and of estimating the elevation of bal¬ 
loons. 

Emily. And is no inconvenience experienced from the 
thinness of the air in such elevated situations? 

Mrs. B Oh, yes; frequently. It is sometimes oppres¬ 
sive, from being insufficient for respiration; and the expan¬ 
sion which takes place in the more dense air contained with¬ 
in the body is often painful: it occasions distension, and 
sometimes causes the bursting of the smaller blood-vessels in 
the nose and ears. Besides, in such situations, you are more 
exposed both to heat and cold; for though the atmosphere is 
itself transparent, its lower regions abound with vapours and 
exhalations from the earth, which float in it, and act in 
some degree as a covering, which preserves us equally from 
the intensity of the sun’s rays, and from the severity of the 
cold. 

Caroline . Pray, Mrs. B., is not the thermometer con¬ 
structed on the same principles as the barometer? 

Mrs. B. Not at ail. The rise and fall of the fluid in 
the thermometer is occasioned by the expansive power of 
heat, and the condensation produced by cold: the air has no 
access to it. An explanation of it would, therefore, be ir¬ 
relevant to our present subject. 

Emily. I have been reflecting, that since it is the weight 
of the atmosphere which supports the mercury in the tube 
of a barometer, it would support a column of any other fluid 
in the same manner. 

Mrs. B. Certainly; but as mercury is heavier than all 
other fluids, it will support a higher column of any other 
fluid; for two fluids are in equilibrium, when their height 
varies inversely as their densities. We find the weight of 
the atmosphere is equal to sustaining a column of water, for 
instance, of no less than 32 feet above its level. 

Caroline. The weight of the atmosphere, i? then, as 
great as that of a body of water the depth of 32 feet? 

Mrs. B. Precisely; for a column of air of the height of 
the atmosphere is equal to a column of water of 32 feet, or 
one of mercury of 28 inches. 

The common pump is constructed on this principle. By 


MECHANICAL PROPERTIES OF AIR. 161 

the act of pumping, the pressure of the atmosphere is taken 
off the water, which, in consequence, rises. 

The body of a pump consists of a large tube or pipe, 
whose lower end is immersed in the water which it is de¬ 
signed to raise. A kind of stopper, called a piston, is fitted 
to this tube, and is made to slide up and down it by means 
of a metallic rod fastened to the centre of the piston. 

Emily. Is it not similar to the syringe, or squirt, with 
which you first draw in, and then force out water? 

Mrs. B. It is; but you know that we do not wish to 
force the water out of the pump, at the same end of the pipe 
at which we draw it in. The intention of a pump is to 
raise water from a spring or well; the pipe is, therefore, 
placed perpendicularly over the water which enters it at the 
lower extremity, and it issues at a horizontal spout towards 
the upper part of the pump. The pump, therefore, is ra¬ 
ther a more complicated piece of machinery than the 
syringe. 

Its various parts are delineated in this figure: (fig. 4. plate 
XIV.) A B is the pipe or body of the pump, P the piston, V 
a valve, or little door in the piston, which opening upwards, 
admits the water to rise through it, but prevents its returning, 
and Y a similar valve in the body ol the pump. 

When the pump is in a state of inaction, the two valves 
are closed by their own weight; but when, by drawing down 
the handle of the pump, the piston ascends, it raises a co¬ 
lumn of air which rested upon it, and produces a vacuum 
between the piston and the lower valve Y, the air beneath 
this valve, which is immediately over the surface of the wa¬ 
ter, consequently expands, and forces its way through it; 
the water, then, relieved from the pressure of the air, as¬ 
cends into the the pump. A few strokes of the handle total¬ 
ly excludes the air from the body of the pump, and fills it 
with water, which, having passed through both the valves, 
runs out at the spout. 

Caroline I understand this perfectly. When the piston 
is elevated, the air and the water successively rise in the 
pump, for the same reason as the mercury rises in the 
barometer. 



162 MECHANICAL PROPERTIES OF AIR. 

Emily. I thought that water was drawn up into a pump, 
by suction, in the same manner as water may be sucked 
through a straw. 

Mrs. B. It is so, into the body of the pump; for the 
power of suction is no other than that of producing a vacu¬ 
um over one part of the liquid, into which vacuum the 
liquid is forced, by the pressure of the atmosphere on ano¬ 
ther part. The action of sucking through a straw, consists 
in drawing in and confining the breath, so as to produce a 
vacuum in the mouth; iu consequence of which, the air 
within the straw rushes into the mouth, and is followed by 
the liquid, into which the lower end of the straw is im¬ 
mersed. The principle, you see, is the same, and the only 
difference consists in the mode of producing a vacuum. In 
suction, the muscular powers answer the purpose of the pis¬ 
ton and valve. 

Emily. Water can not, then, be raised by a pump above 
32 feet; for the pressure of the atmosphere will not sustain 
a column of water above that height. 

Mrs. B. I beg your pardon. It is true that there must 
never be so great a distance as 32 feet from the level of the 
water in the well, to the valve in the piston, otherwise the 
water would not rise through that valve; but when once the 
water has passed that opening, it is no longer the pressure 
of air on the reservoir which makes it ascend; it is raised by 
lifting it up, as you would raise it in a bucket, of which the 
piston formed the bottom. This common pump, is, there- 
tore, called the sucking, or lifting-pump, as it is constructed 
on both these principles. There is another sort of pump, 
called the forcing-pump: it consists of a forcing power ad¬ 
ded to the sucking part of the pump. This additional pow¬ 
er is exactly on the principle of the syringe: by raising the 
piston you draw the water into the pump, and by descending 
it you force the water out. 

Caroline. But the water must be forced out at the up¬ 
per part of the pump; and I can not conceive how that can 
be done by descending the piston. 

Mrs . B. Figure 5. pi. XIV will explain the difficulty. 
The large pipe A B represents the sucking part of the pump, 


MECHANICAL PROPERTIES OF AIR. 


163 


which differs from the lifting-pump, only in its piston P be¬ 
ing unfurnished with a valve, in consequence of which the 
water can not rise above it. When, therefore, the piston 
descends, it shuts the vaive Y, and forces the water (which 
has no other vent) into the pipe D: this is likewise furnish¬ 
ed with a valve V, which opening outwards, admits the wa¬ 
ter, but prevents its return. 

The water is thus first raised in the pump, and then for¬ 
ced into the pipe, by the alternate ascending and descend¬ 
ing motion of the piston, after a few strokes of the handle 
to fill the pipe, from whence the water issues at the spout. 

It is now time to conclude our lesson. When next we 
meet, I shall give you some account of wind, and of sound, 
which will terminate our observations on elastic fluids. 

Caroline. And I shall run into the garden, to have the 
pleasure of pumping, now that I understand the construc¬ 
tion of a pump. 

Mrs . B. And, to-morrow, I hope you will be able to tell 
me, whether it is a forcing or a common lifting pump. 



CONVERSATION XIII. 


ON WIND AND SOUND. 


#F "WIND IN GENERAL.—OF THE TRADE WIND.—OF THE PERIODICAL 

TRADE WINDS.-OF THE AERIAL TIDES.-OF SOUNDS IN GENERAL.—OF 

SONOROUS BODIES.—OF MUSICAL SOUNDS.—OF CONCORD OR HARMON T, 

AND HARMON*. 

MRS. B. 

Well, Caroline, have you ascertained what kind of 
pump you have in your garden? 

Caroline. I think it must be merely a lifting-pump, be¬ 
cause no more force is required to raise the handle than is 
necessary to lift its weight; and in a forcing pump, by rais¬ 
ing the handle, you force the water into the smaller pipe, 
and the resistance the water offers must require an exertion 
of strength to overcome it. 

Mrs , B. I make no doubt you are right; for lifting pumps 
being simple in their construction, are by far the most com¬ 
mon. 

I have promised to day to give you some account of the 
nature of wind. Wind is nothing more than the motion 
of a stream or current of air, generally produced by a partial 
change of temperature in the atmosphere; for when any one 
part is more heated than the rest, that part is rarefied; the 
equilibrium is destroyed, and the air in consequence rises. 
When this happens, there necessarily follows a motion of the 
surrounding air towards that part, in order to restore it; this 
spot, therefore, receives winds from every quarter. Those 


02s WIND AND SOUND, 166 

who live to the north of it experience a north wind; those 
to the south, a south wind:—do you comprehend this? 

Caroline. Perfectly. But what sort of weather must 
those people hare, who live on the spot where these w r inds 
meet and interfere? 

Mrs. B. They have turbulent and boisterous weather, 
whirlwinds, hurricanes, rain, lightning, thunder, &c. This 
stormy Weather occurs most frequently in the torrid zone, 
where the heat is greatest: the air being more rarefied there 
than in any other part of the globe, is lighter, and conse¬ 
quently ascends; whilst the air about the polar regions is 
continually flowing from the poles, to restore the equilibrium. 

Caroline. This motion of the air would produce a regu¬ 
lar and constant north wind to the inhabitants of the north¬ 
ern hemisphere; and a south wind to those of the southern 
hemisphere, and continual storms at the equator, where these 
two adverse winds would meet. 

Mrs. B. These winds do not meet, for they each change 
their direction before they reach the equator. The sun, in 
moving over the equatorial regions from east to west, rarefies 
the air as it passes, and causes the denser eastern air to flow 
westwards, in order to restore the equilibrium, thus produ¬ 
cing a regular east wind about the equator. 

Caroline. The air from the west, then constantly goes 
to meet the sun, and repair the disturbance which his beams 
have produced in the equilibrium of the atmosphere. But 
fi wonder how you will reconcile these various winds, Mrs. 
B .: you first led me to suppose there was a constant strug¬ 
gle between opposite winds at the equator, producing storm 
and tempest; but now I hear of one regular invariable wind, 
which must naturally be attended by calm weather. 

Emily I think I comprehend it: do not these winds 
from the north and south combine with the easterly wind 
about the equator, and form what are called the trade winds? 

Mrs. B Just so, my dear. The composition of the two 
winds north and east, produces a constant north-east wind; 
and that of the two winds south and east, produces a regu¬ 
lar south'east wind; these winds extend to about thirty de¬ 
grees on each side of the equator, the regions further distant 
* 15 


166 


ON WIND AND SOUND. 


from it experiencing only their respective north and south 
winds. 

Caroline. But Mrs. B., if the air is constantly flowing 
from the poles to the torrid zone, there must be a deficiency 
of air in the polar regions? 

Mrs. B. The light air about the equator, which ex¬ 
pands and rises into the upper regions of the atmosphere, 
ultimately flows from thence back to the poles, to restore 
the equilibrium: if it were not for this resource, the polar 
atmospheric regions would soon be exhausted by the stream 
of air, which, in the lower strata of the atmosphere, they are 
constantly sending towards the equator. 

Caroline. There is then a sort of circulation of air in 
the atmosphere; the air in the lower strata flowing from the 
poles towards the equator, and in the upper strata, flowing 
back from the equator towards the poles. 

Mrs. B. Exactly: I can show you an example of this 
circulation on a smaller scale. The air of this room being 
more rarefied than the external air, a wind or current of air 
is pouring in from the crevices of the windows and doors, 
to restore the equilibrium; but the light air with which the 
room is filled must find some vent, in order to make way for 
the heavy air that enters. If you set the door a-jar, and 
bold a candle near the upper part of it, you will find that 
the flame will be blown outwards, showing that there is a 
current of air flowing out from the upper part of the room.—* 
Now place the candle on the floor close by the door, and 
you will perceive, by the inclination of the flame, that there 
is also a current of air setting into the room. 

Caroline. It is just so; the upper current is the w r arm 
light air, which is driven out to make way for the stream 
of cold dense air which enters the room lower down. 

Emily. I have heard, Mrs. B., that the periodical winds 
are not so regular on land as at sea: what is the reason of 
that? 

Mrs. B. The land reflects into the atmosphere a much 
greater quantity of the sun’s rays than the water; therefore, 
that part of the atmosphere which is over the land, is more 
heated and rarefied than that which is over the sea: this oc- 


ON WIND AND SOUND. 


167 


casions the wind to set in upon the land, as we find that it 
regularly does on the coast of Guinea, and other countries in 
the torrid zone. 

Emily. I have heard much of the violent tempests occa* 
sioned by the breaking up of the monsoons; are not they also 
regular trade-winds? 

Mrs . B. They are called periodical trade-winds, as they 
change their course every half year. This variation is pro¬ 
duced by the earth’s annual course round the sun, when the 
north pole is inclined towards that luminary one half of the 
year, the south pole the other half. During the summer of 
the northern hemisphere, the countries of Arabia, Persia, 
India, and China, are much heated, and reflect great quanti¬ 
ties of the sun’s rays into the atmosphere, by which it be¬ 
comes extremely rarefied, and the equilibrium consequently 
destroyed. In order to restore it the air from the equatorial 
southern regions, where it is colder, (as well as from the 
colder northern parts,) must necessarily have a motion to¬ 
wards those parts. The current of air from the equatorial 
regions produces the trade-winds for the first six months, in 
all the seas between the heated continent of Asia, and the 
equator. The other six months, when it is summer in the 
southern hemisphere, the ocean and countries towards the 
southern tropic are most heated, and the air over those parts 
more rarefied: then the air about the equator alters its course, 
and flows exactly in an opposite direction. 

Caroline. This explanation of the monsoons is very cu¬ 
rious; but what does their breaking up mean? 

Mrs. B. It is the name given by sailors to the shifting 
of the periodical winds; they do not change their course sud¬ 
denly, but by degrees, as the sun moves from one hemisphere 
to the other: this change is usually attended by storms and 
hurricanes, very dangerous for shipping; so that those seas 
are seldom navigated at this season of the equinox. 

Emily . I think I understand the winds in the torrid zone 
perfectly well; but what is it that occasions the great variety 
of winds which occur in the temperate zones? for, accord¬ 
ing to your theory, there should be only north and south 
winds in those climates. 


168 


ON WIND AND SOUND. 


Mrs. B. Since so large a portion of the atmosphere as 
is over the torrid zone is in continued agitation, these agita¬ 
tions in au elastic fluid, which yields to the slightest impres¬ 
sion, miust extend every way to a great distance; the air, 
therefore, in all climates, will suffer more or less perturba¬ 
tion, according to the situation of the country, the position 
of mountains, valleys, and a variety of other causes: hence 
it is easy to conceive, that almost every climate must be 
liable to variable winds. 

On the sea-shore, there is almost always a gentle sea- 
breeze setting in on the land on a summer’s evening, to re¬ 
store the equilibrium which had been disturbed by reflec¬ 
tions from the heated surface of the shore during the day; 
and when night has cooled the land, and condensed the air, 
we generally find it towards morning, flowing back towards 
the sea. 

Caroline . I have observed, that the wind, whichever 
way it blows, almost always falls about sun-set. 

Mrs. B. Because the rarefaction of air in the particular 
spot.which produces the wind, diminishes as the sun de¬ 
clines, and consequently the velocity of the wind abates. 

Emily. Since the air is a gravitating fluid, is it not af¬ 
fected by the attraction of the moon and the sun, in the same 
manner as the waters? 

Mrs. B. Undoubtedly; but the aerial tides are as much 
greater than those of water, as the density of w r ater exceeds 
ihat of air, which, as you may reGoilect, we found to be 
about 800 to L 

Caroline. What a prodigious protuberance that must oc¬ 
casion ! How much the weight ef such a column of air must 
raise the mercury in the barometer! 

Emily. As this enormous tide of air is drawn up and 
supported, as it were by the moon, its weight and pressure, 
I should suppose, would be rather diminished than in¬ 
creased ? 

Mrs. B. The weight of the atmosphere is neither in¬ 
creased nor diminished by the aerial tides. The moon’s 
attraction augments the bulk as much as it diminishes the 
weight of the column of air; these effects, therefore, coun- 


ON WIND AND SOUND. 169 

terbalancing each other, the serial tides do not affect the 
barometer. 

Caroline. I do not quite understand that. 

Mrs. B. Let us suppose that the additional bulk of air 
at high tide raises the barometer one inch; and on the other 
hand, that the support which the moon’s attraction affords 
the air diminishes its weight or pressure, so as to occasion 
the mercury to fall one inch; under these circumstances the 
mercury must remain stationary. Thus, you see, that we 
can never be sensible of serial tides by the barometer, on 
account of the equality of pressure of the atmosphere, what¬ 
ever be its height. 

The existence of aerial tides is not, however, hypotheti¬ 
cal; it is proved by the effect they produce on the apparent 
position of the heavenly bodies; but this I can not explain 
to you, till you understand the properties of light. 

Emily. And when shall we learn them? 

Mrs. B. I shall first explain to you the nature of sound, 
which is intimately connected with that of air; and I think 
at our next meeting we may enter upon the subject of optics. 

We have now considered the effects produced by the 
wide and extended agitation of the air; but there is another 
kind of agitation of which the air is susceptible—a sort of 
vibratory trembling motion, which, striking on the drum of 
the ear, produces sound. 

Caroline. Is not sound produced by solid bodies? The 
voice of animals, the ringing of bells, musical instruments, 
are all solid bodies. I know of no sound but that of the 
wind which is produced by the air. 

Mrs. B. Sound, I assure you, results from a tremulous 
motion of the air; and the sonorous bodies you enumerate, 
are merely the instruments by which that peculiar species of 
motion is communicated to the air. 

Caroline. What! when I ring this little bell, is it the air 
that sounds, and not the bell? 

Mrs. B. Both the bell and the air are concerned in the 
production of sound. But sound, strictly speaking, is a per¬ 
ception excited in the mind by the motion of the air on the 
nerves of the ear; the air, therefore, as well as the sonorous 
15 * 


no 


ON WIND AND SOUND, 


bodies which put it in motion, is only the cause of sound, 
the immediate effect is produced by the sense of hearing: 
for without this sense, there would be no sound. 

Emily. I can with difficulty conceive that. A person 
born deaf, it is true, has no idea of sound, because he hears 
none; yet that does not prevent the real existence of sound, 
as all those who are not deaf can testify. 

Mrs. B. I do not doubt the existence of sound to all 
those who possess the sense of hearing; but it exists neither 
in the sonorous body nor in the air, but in the mind of the 
person whose ear is struck by the vibratory motion of the 
air, produced by a sonorous body. 

To convince you that sound does not exist in sonorous 
bodies, but that air or some other vehicle is necessary to its 
production, endeavour to ring the little bell, after I have 
suspended it under a receiver in the air-pump, from which 
I shall exhaust the air. 

Caroline. This is indeed very strange: though I agitate 
it so violently, it does not produce the least sound. 

Mrs. B. By exhausting the receiver, I have cut off the 
communication between the air and the bell; the latter, 
therefore, can not impart its motion to the air 

Caroline. Are you sure that it is not the glass, which 
covers the bell, that prevents our hearing it? 

Mrs. B. That you may easily ascertain by letting the 
air into the receiver, and then ringing the bell. 

Caroline . Very true: I can hear it now almost as loud 
as if the glass did not cover it; and I can no longer doubt 
but that air is necessary to the production of sound. 

Mrs. B . Not absolutely necessary, though by far the 
most common vehicle of sound. Liquids, as well as air, are 
capable of conveying the vibratory motion of a sonorous 
body to the organ of hearing; as sound can be heard under 
water. Solid bodies also convey sound, as I can soon con¬ 
vince you by a very simple experiment. I shall fasten this 
string by the middle round the poker; now raise the poker 
from the ground by the two ends of the string and hold one 
to each of your ears:—I shall now strike the peker with a 
key, and you will find that the sound is conveyed to the car 



ON WIND AND SOUND. 171 

by means of the strings, in a much more perfect manner 
than if it had no other vehicle than the air. 

Caroline. That it is, certainly, for I am almost stunned 
by the noise. But what is a sonorous body, Mrs. B>? for all 
bodies are capable of producing some kind of sound by the 
motion they communicate to the air. 

Mrs . B. Those bodies are called sonorous, which pro* 
duce clear, distinct, regular and durable sounds, such as a 
bell, a drum, musical strings, wind-instruments, &c. They 
owe this property to their elasticity; for an elastic body, after 
having been struck, not only returns to its former situation, 
but having acquired momentum by its velocity, like the pen¬ 
dulum, it springs out on the opposite side. If I draw the 
string A B, which is made fast at both ends to C, it will 
not only return to its original position, but proceed onwards 
to D. 

This is its first vibration, at the end of which it will re¬ 
tain sufficient velocity to bring it to E, and back again to F, 
which constitutes its second vibration; the third vibration 
wili carry it only to G and II, and so on till the resistance 
of the air destroys its motion. 

The vibration of a sonorous body gives a tremulous mo¬ 
tion to the air around it, very similar to the motion commu¬ 
nicated to smooth water when a stone is thrown into it 
This first produces a small circular wave around the spot in 
/ which the stone falls; the wave spreads and gradually com¬ 
municates its motion to the adjacent waters, producing simi¬ 
lar waves to a considerable extent. The same kind of waves 
are produced in the air by the motion of a sonorous body, 
but with this difference, that as air is an elastic fluid, the 
motion does not consist of regularly extending waves, but of 
vibrations, and are composed of a motion forwards and back¬ 
wards, similar to those of the sonorous body. They differ 
also in the one taking place in a plane, the other in all di¬ 
rections. The aerial undulations being spherical. 

Emily. But if the air moves backwards as well as for¬ 
wards, how can its motion extend so as to convey sound to 
a distance? 


m 


ON WIND AND SOUND. 


Mrs. B. The first sphere of undulations which are pro- 
duced immediately around the sonorous body, by pressing 
against the contiguous air, condenses it. The condensed 
air, though impelled forward by the pressure, re-acts on the 
first set of undulations, driving them back again. The se¬ 
cond set of undulations which have been put in motion, in 
their turn communicate their motion, and are themselves 
driven back by reaction. Thus there is a succession of waves 
in the air, corresponding with the succession of waves in the 
water. 

Caroline. The vibrations of sound must extend much 
further than the circular waves in water, since sound is 
conveyed to'a great distance. 

Mrs. B. The air is a fluid so much less dense than wa¬ 
ter, that motion is more easily Communicated to it. The re¬ 
port of a cannon produces vibrations of the air which extend 
to several miles around. 

Emily . Distant sound takes some time to reach us, 
since it is produced at the moment the cannon is fired; and 
we see the light of the flash long before we hear the report. 

Mrs. B. The air is immediately put in motion by the 
firing of a cannon; but it requires time for the vibrations to 
extend to any distant spot. The velocity of sound is com¬ 
puted to be at the rate of 1142 feet in a second. 

Caroline. With W'hat astonishing rapidity the vibrations 
must be communicated! But the velocity of sound varies, 
I suppose, with that of the air which conveys it. If the 
wind sets towards us from the cannon we must hear the re¬ 
port sooner than if it set the other way. 

Mrs. B. The direction of the wind makes less differ¬ 
ence in the velocity of sound than you would imagine. If 
the wind sets from us, it bears most of the aerial waves 
away, and renders the sound fainter; but it is not very con¬ 
siderably longer in reaching the ear than if the wind blew 
towards us. This uniform velocity of sound enables us to 
determine the distance of the object from which it proceeds; 
as that of a vessel at sea firing a cannon, or that of a thun¬ 
der cloud. If we do not hear the thunder till half a minute 


ON WIND AND SOUND. 173 

after we see the lightning, we conclude the cloud to be at 
the distance of six miles and a half. 

Emily. Pray how is the sound of an echo produced? 

Mrs. B. When the aerial vibrations meet with an ob¬ 
stacle, having a hard and regular surface, such as a wall, or 
rock, they are reflected back to the ear, and produce the 
same sound a second time; but the sound will then appear 
to proceed from the object by which it is reflected. If the 
vibrations fall perpendicularly on the obstacle, they are re¬ 
flected back in the same line; if obliquely, the sound returns 
obliquely in the opposite direction, the angle of reflection 
being equal to the angle of incidence. 

Caroline. Oh, then, Emily, I now understand why 
the echo of my voice behind our house is heard so much 
plainer by you than it is by me, when we stand at the op¬ 
posite ends of the gravel walk. My voice, or rather, 1 should 
say, the vibrations of air it occasions, fail obliquely on the 
wall of the house* and are reflected by it to the opposite end 
of the gravel walk. 

Emily. Very true; and we have observed, that when 
we stand in the middle of the walk, opposite the house, the 
echo returns to the person who spoke. 

Mrs. B. Speaking-trumpets are constructed on the prin¬ 
ciple of the reflection of sound. The voice, instead of be¬ 
ing diffused in the open air, is confined within the trumpet; 
and the vibrations, which spread and fall against the sides 
of the instrument, are reflected according to the angle of in¬ 
cidence, and fali into the direction of the vibrations which 
proceed straight forwards. The whole of the vibrations are 
thus collected into a focus; and if the ear be situated in or 
near that spot, the sound is prodigiously increased. Figure 
7. plate XIV. will give you a clearer idea of the speaking 
trumpet: the reflected rays are distinguished from those of 
incidence, by being dotted; and they are brought to a focus 
at F. The trumpet used by deaf persons acts on the same 
principle; but as the voice enters the trumpet at the large, 
instead of the small end of the instrument, it is not so much 
confined, nor the sound so much increased. 


174 


ON WIND AND SOUND. 


Emily. Are the trumpets used as musical instruments, 
also constructed on this principle? 

Mrs. B. So far as their form tends to increase the sound, 
they are; but, as a musical instrument, the trumpet becomes 
itself the sonorous body, which is made to vibrate by blow¬ 
ing into it, and communicates its vibrations to the air. 

f will attempt to give you, in a few words, some notion 
of the nature of musical sounds, which, as you are fond of 
music, must be interesting to you. 

It a sonorous body be struck in such a manner, that its 
vibrations are all performed in regular times, the vibrations 
of the air will correspond with them; and striking in the 
same regular manner on the drum of the ear, will produce 
the same uniform sensation on the auditory nerve, and ex¬ 
cite the same uniform idea in the mind; or, in other words, 
we shall here one musical tone. 

But if the vibrations of the sonorous body are irregular, 
there will necessarily follow a confusion of aerial vibrations; 
for a second vibration may commence before the first is fin¬ 
ished, meet it half way on its return, interrupt it in its 
course, and produce harsh jarring sounds, which are called 
discords. 

Emily. But each set of these irregular vibrations, if re¬ 
repeated at equal intervals, would, I suppose, produce a mu¬ 
sical tone? It is only their irregular succession which makes 
them interfere, and occasions discord. 

Mrs. B. Certainly, The quicker a sonorous body vi¬ 
brates, the more acute, or sharp, is the sound produced. 

Caroline. But if I strike any one note of the piano-forte 
repeatedly, whether quickly or slowly, it always gives the 
same tone. 

Mrs. B. Because the vibrations of the same string, at 
the same degree of tension, are always of a similar duration. 
The quickness or slowness of the vibrations relate to the 
single tones, not to the various sounds which they may 
compose by succeeding each other. Striking the note in 
quick succession, produces a more frequent repetition of 
the tone, but does not increase the velocity of the vibrations 
of the string. 


ON WIND AND SOUND. 


175 


The duration of the vibrations of strings or chords, de¬ 
pends upon their length, their thickness or weight, and their 
degree of tension: thus, you find, the low bass notes are 
produced by long, thick loose strings; and the high treble 
notes by short, small, and tight strings. 

Caroline. Then the different length and size of the 
strings of musical instruments, serves to vary the duration 
of the vibrations, and consequently, the acuteness of gravity 
of the notes? 

Mrs. B. Yes. Among the variety of tones, there are 
some which, sounded together, please the ear, producing 
what we call harmony, or concord. This arises from the 
agreement of the vibrations’of the two sonorous bodies; so 
that some of the vibrations of each strike upon the ear at 
the same time. Thus, if the vibrations of two strings are 
performed in equal times, the same tone is produced by both, 
and they are said to be in unison. 

Emily. Now, then, I understand why, when I tune my 
harp in unison with the piano-forte, I draw the strings tight¬ 
er if it is too low, or loosen them if it is at too high a pitch: 
it is in order to bring them to vibrate, in equal times, with 
the strings of the piano-forte. 

Mrs. B. But concord, you know, is not confined to 
unison; for two different tones harmonize in a variety of 
cases. If the vibrations of one string (or sonorous body 
whatever) vibrate in double the time of another, the second 
vibration of the latter will strike upon the ear at the same 
instant as the first vibration of the former; and this is the 
concord of an octave. 

If the vibrations of two strings are as two to three, the se¬ 
cond vibration of the first corresponds with the third vibra¬ 
tion of the latter, producing the harmony called a fifth. 

Caroline. So, then, when I strike the key-note with its 
fifth, I hear every second vibration of one, and every third 
of the other at the same time? 

Mrs. B. Yes; and the key-note struck with the fourth 
is likewise a concord, because the vibrations are as three to 
four. The vibrations of a major third with the key-note, 
are as four to five; and those of a minor third, as five to six. 



176 


ON WIND AND SOUND. 


There are other tones, which, though they can not be 
struck together without producing discord, if struck succes¬ 
sively, give us the pleasure which is called melody. Upon 
these general principles the science of music is found; but I 
am not sufficiently acquainted with it to enter any further 
into it ; 

We shall now, therefore, take leave of the subject of 
sound; and, at our next interview, enter upon that of optics, 
in which we shall consider the nature of vision, light, and 
colours. 


CONVERSATION XIV. 


ON OPTICS. 

OF LUMINOUS, TRANSPARENT, AND OPAQ.UJ3 BODIES.-OF THE RADIATION 

OF LIGHT.-OF SHADOWS.-OF THE REFLECTION OF LIGHT.-OPAQ.UE 

BODIES SEEN ONLY BY REFLECTED LIGHT.—VISION EXPLAINED.— 

CAMERA OBSCURA.—IMAGE OF OBJECTS ON THE RETINA. 

CAROLINE. 

I long to begin Our iesson to day, Mrs. B., for I expect 
that it will be very entertaining. 

Mrs. B. Optics is certainly one of the most interesting 
branches of Natural Philosophy, but not one of the easiest to 
understand; I must therefore beg that you will give me the 
whole of your attention, 

I shall first inquire, whether you comprehend the meaning 
of a luminous body , an opaque body , and a transparent body . 

Caroline. A luminous body is one that shines; a'n 
opaque.... 

Mrs. B. Do not proceed to the second, until we have 
agreed upon the definition of the first. All bodies that shine 
are not luminous; for a luminous body is one that shines by 
its own light, as the sun, the fire, a candle, &c. 

Emily. Polished metal then, when it shines with so 
much brilliancy, is not a luminous body? 

Mrs. B. No, for it would be dark if it did not receive 
light from a luminous body; it belongs, therefore, to the class 
of opaque or dark bodies, which comprehend all such as 
are neither luminous nor will admit the light to pass through 
them. 


16 


ON OPTICS. 


ns 

Emily. And transparent bodies, are those which admit 
the light to pass through them; such as glass and water. 

Mrs. B. You are right. Transparent or pellucid bodies, 
are frequently called mediums; and the rays of light which 
pass through them, are said to be transmitted by them. 

Light, when emanated from the sun, or any other lumin¬ 
ous body, is projected forwards in straight lines in every 
possible direction; so that the luminous body is not only the 
general centre from whence all the rays proceed; but every 
point of it may be considered as a centre which radiates 
light in every direction. (Fig. 1, plate XY.) 

Emily. But do not the rays which are projected in dif¬ 
ferent directions, and cross each other, interfere and impede 
each other’s course? 

Mrs . B. Not at all. The particles of light are so ex¬ 
tremely minute, that they are never known to interfere with 
each other. A ray of light is a single line of light projected 
from a luminous body; and a pencil of rays, is a collection of 
rays, proceeding from any one point of a luminous body, 
as fig. 2. 

Caroline. Is light then a substance composed of par¬ 
ticles like other bodies? 

Mrs. B. That is a disputed point, upon which I can not 
pretend to decide. In some respects, light is obedient to 
the laws which govern bodies; in others, it appears to be 
independent of them: thus though its course is guided by the 
laws of motion, it does not seem to be influenced by those 
of gravity. It has never been discovered to have weight, 
though a variety of interesting experiments have been made 
with a view of ascertaining that point; but we are so igno¬ 
rant of the intimate nature of light, that an attempt to inves¬ 
tigate it would lead us into a labyrinth of perplexity, if not 
of error; we shall therefore confine our attention to those 
properties of light which are well ascertained. 

Let us return to the examination of the effects of tbe radia¬ 
tion of light from a luminous body. Since the rays of light 
are projected in straight lines, when they meet with an 
opaque body through which they are unable to pass, they 






































































































































































































































































































































































































































































ON OPTICS. 179 

are stopped short in their course; for they can not move iu 
a curve line round the body. 

Caroline, No, certainly; for it would require some other 
force besides that of projection, to produce motion in a curve 
line. 

Mrs, B. The interruption of the rays of light, by the 
opaque body, produces, therefore, darkness on the opposite 
side of it; and if this darkness fall upon a wall, a sheet of 
paper, or any object whatever, it forms a shadow. 

Emily . A shadow then is nothing more than darkness 
produced by the intervention of an opaque body, which pre¬ 
vents the rays of light from reaching an object behind the 
opaque body. 

Caroline . Why then are shadows of different degrees of 
darkness; for I should have supposed from your definition of 
a shadow, that it would have been perfectly black? 

Mrs, B. It frequently happens that a shadow is produ- 
duced by an opaque body interrupting the course of the rays 
from one luminous body, while light from another reaches 
the space where the shadow is formed, in which case the 
shadow is proportionally fainter. This happens if the opaque 
body be lighted by two candles: if you extinguish one 
of them, the shadow will be both deeper and more distinct. 

Caroline . But yet it will not be perfectly dark. 

Mrs. B. Because it is still slightly illuminated by light 
reflected from the walls of the room, and other surrounding 
objects. 

You must observe, also, that when a shadow is produced 
by the interruption of rays from a single luminous body, the 
darkness is proportioned to the intensity of the light. 

Emily. I should have supposed the contrary; for as the 
light reflected from surrounding objects on the shadow, must 
be in proportion to the intensity of the light, the stronger 
the light, the more the shadow will be illumined. 

Mrs. B. Your remark is perfectly just; but as we have 
no means of estimating the degrees of light and of darkness 
but by comparison, the strongest light will appear to produce 
the deepest shadow. Hence a total eclipse of the sun occa- 


180 


ON OPTICS. 


sions a more sensible darkness (ban mid-night, as it is imme¬ 
diately contrasted with the strong light of noon-day.. 

Caroline. The reappearance of the sun after an eclipse, 
must by the same contrast be remarkably brilliant. 

Mrs. B. Certainly. There are several thing to be, ob¬ 
served in regard to the form and extent of shadows. If the 
luminous body A (fig. 3.) is larger than the opaque body 
B, the shadow will gradually diminish in size, till it termi¬ 
nate in a point. 

Caroline. This is the case with the shadows of the earth 
and the moon, as the sun which illumines them, is larger 
than either of those bodies. And why is it not the case 
with the shadows of terrestrial objects, which are equally 
illuminated by the sun? But their shadows, far from dimin¬ 
ishing, are always larger than the object, and increase with 
the distance from it. 

Mrs. B, In estimating the effect of shadows, we must 
consider the apparent not the real dimensions of the luminous 
body; and in this point of view, the sun is a small object 
compared with the generality of the terrestrial bodies which 
it illumines: and when the luminous body is less than the 
opaque body, the shadow will increase with the distance to 
infinity. All objects, therefore, which are apparently larger 
than the sun, cast a magnified shadow. This will be best 
exemplied, by observing the shadow of an object lighted by 
a candle. 

Emily. I have often noticed, that the shadow of my fig¬ 
ure against the wall, grows larger as it is more distant from 
me, which is owing, no doubt, to the candle that shines on 
me being much smaller than myself? 

Mrs. B. Yes. The shadow of a figure A, (fig. 4.) va¬ 
ries in size, according to the distance of the several surfaces 
B C D E, on which it is described. 

Caroline. I have observed, that two candles produce 
two shadows from the same object; whilst it would appear, 
from what you said, that they should rather produce only 
half a shadow, that is to say, a very faint one. 

Mrs. B. The number of lights (in different directions) 
while it decreases the intensity of the shadow, increases their 


ON OPTICS. 


131 


number, which always corresponds with that of the lights; 
for each light makes the opaque body cast a different shadow, 
as illustrated by fig. 5, It represents a ball A, lighted by 
three candles B, C, D, and you observe the light B produ¬ 
ces the shadow 6, the light C the shadow c, and the light D 
the shadow d. 

Emily. I think we now understand the nature of sha¬ 
dows very well; but pray what becomes of the rays of light 
which opaque bodies arrest in their course, and the inter¬ 
ruption of which is the occasion of shadows? 

Mrs. B. Your question leads to a very important pro¬ 
perty of light, Reflection. When rays of light encounter an 
opaque body, which they can not traverse, part of them are 
absorbed by it, and part are reflected, and rebound just as 
an elastic ball which is struck against a wall. 

Emily. And is light in its reflection governed by the 
same laws as solid elastic bodies? 

Mrs. B. Exactly. If a ray oflight fall perpendicularly 
on an opaque body, it is reflected back in the same line, to¬ 
wards the point whence it proceeded. If it fall obliquely, 
it is reflected obliquely, but in the opposite direction; the 
angle of incidence being equal to the angle of reflection. 
You recolieGt that law in mechanics? 

Emily. Oh yes, perfectly. 

Mrs. B. If you will shut the shutters, we shall admit a 
ray of the sun’s light through a very small aperture, and I 
can show you how it is reflected. I now hold this mirror, 
so that the ray shall fall perpendicularly upon it. 

Caroline. I see the ray which tails upon the mirror, but 
not that which is reflected by it. 

Mrs. B. Because its reflection is directly retrograde. 
The ray of incidence and that of reflection both being in 
the' same line, though in opposite directions, are confounded 
together. 

Emily. The ray then which appears to us single, is really 
double, and is composed of the incident ray proceeding to 
the mirror, and of the reflected ray returning from the mirror. 

Mrs. B . Exactly so. We shall now separate them by 
holding the mirror M, (fig. 6.) in such a manner, that the 
16 * 


182 


ON OPTICS. 


incident ray A B shall fall obliquely upon it—you see the 
reflected ray B C, is marching off in another direction. If 
we draw a line from the point of incidence B, perpendicular 
to the mirror, it will divide the angle of incidence from the 
angle of reflection, and you will see that they are equal. 

Emily. Exactly; and now that you hold the mirror so, 
that the ray falls more obliquely upon it, it is also reflected 
more obliquely, preserving the equality of the angles of in¬ 
cidence and reflection. 

Mrs. B. It is by reflected rays only that we see opaque 
objects. Luminous bodies send rays of light immediately 
to our eyes, but the rays which they send to other bodies are 
invisible to us, and are seen only when they are reflected or 
transmitted by those bodies to our eyes. 

Emily. But have we not just seen the ray of light in its 
passage from the sun to the mirror, and its reflections? yet 
in neither case were those rays in a direction to enter our 
eyes. 

Mrs. B. No. What you saw was the light reflected to 
your eyes by small particles of dust floating in the air, and 
on which the ray shone in its passage to and from the mirror. 

Caroline. Yet I see the sun shining on that house yon¬ 
der, as clearly as possible. 

Mrs. B. Indeed you can not see a single ray which 
passes from the sun to the house; you see no rays but those 
which enter your eyes; therefore it is the rays which are re¬ 
flected by the house to you, and not those which proceed 
from the sun to the house, that are visible to you. 

Caroline Why then does one side of the house appear 
to be in sunshine, and the other in the shade? for if 1 can 
not see the sun shine upon it, the whole of the house should 
appear in the shade. 

Mrs. B. That side of the house which the sun shines 
upon, reflects more vivid and luminous rays than the side 
which is in shadow, for the latter is illumined only by rays 
reflected upon it by other objects, these rays are therefore 
twice reflected before they reach your sight; and as light is 
more or less absorbed by the bodies it strikes upon, eveiy 
time a ray is reflected its intensity is diminished. 


ON OPTICS, 


183 

Caroline. Still I can not reconcile myself to the idea, 
that we do not see the sun’s rays shining on objects, but 
only those which objects reflect to us. 

Mrs. B. I do not, however, despair of convincing you 
of it. Look at that large sheet of water, can you tell why 
the sun appears to shine on one part of it only? 

Caroline. No, indeed; for the whole of it is equally ex¬ 
posed to the sun. This partial brilliancy of water has often 
excited my wonder; but it has struck me more particularly 
by moon-light. I have frequently observed a vivid streak 
of moonshine on the sea, while the rest of the water remain¬ 
ed in deep obscurity, and yet there was no apparent obstacle 
to prevent the moon from shining on every part of the water 
equally. 

Mrs. B. By moon-light the effect is more remarkable, 
on account of the deep obscurity of the other parts of the wa¬ 
ter; while by the sun’s light the effect is too strong for the 
eye to be able to contemplate it. 

Caroline. But if the sun really shines on every part of 
that sheet of water, why does not every part of it reflect 
rays to my eyes? 

Mrs. B. The reflected rays are not attracted out of their 
natural course by your eyes. The direction of a reflected 
ray, you know, depends on that of the incident ray; the 
sun’s rays, therefore, which fall with various degrees of ob¬ 
liquity upon the water, are reflected in directions equally 
various; some of these will meet your eyes, and you will 
see them, but those which fall elsewhere are invisible to 
you. 

Caroline. The streak of sunshine, then, which we now 
see upon the water, is composed of those rays which by 
their reflection happen to fall upon my eyes? 

Mrs. B. Precisely. 

Emily. But is that side of the house yonder, which ap¬ 
pears to be in shadow, really illuminated by the sun, and 
its rays reflected another way ? 

Mrs. B. No; that is a different case from the sheet of 
water. That side of the house is really in shadow; it is 


184 


ON OPTICS. 


the west side, which the sun can not shine upon till the 
afternoon. 

Emily. Those objects, then, which are illumined by 
reflected rays, and those which receive direct rays from the 
sun, but which do not reflect those rays towards us, appear 
equally in shadow? 

Mrs. B . Certainly; for we see them both illumined by 
reflected rays. That part of the sheet of water, over which 
the trees cast a shadow, by what light do you see it? 

Emily. Since it is not by the sun’s direct rays, it must 
be by those reflected on it from other objects, and which it 
again reflects to us. 

Caroline. But if we see all terrestrial objects by reflect¬ 
ed light, (as we do the moon,) why do they appear so bright 
and luminous? I should have supposed that reflected rays 
would have been dull and faint, like those of the moon. 

Mrs. B. The moon reflects the sun’s light with as much 
vividness as any terrestrial object. If you look at it on a 
clear night, it will appear as bright as a sheet of water, the 
walls of a house, or any object seen by daylight and on 
which the sun shines. The rays of the moon are doubtless 
feeble, when compared with those of the sun; but that would 
not be a fair comparison, for the former are incident, the 
latter reflected rays. 

Caroline. True; and when we see terrestrial objects by 
moon light, the light has been twice reflected, and is conse¬ 
quently proportionally fainter. 

Mrs. B. In traversing the atmosphere, the rays, both of 
the sun and moon, lose some of their light. For though the 
pure air is a transparent medium, which transmits the rays 
of light freely, we have observed, that near the surface of 
the earth it is loaded with vapours and exhalations, by which 
some portion of them are absorbed. 

Caroline. I have often noticed, that an object on the 
summit of a hill appears more distinct than one at an equal 
distance in a valley, or a plain; which is owing, I suppose, 
to the air being more free from vapours in an elevated situ¬ 
ation, and the reflected rays being consequently brighter. 
Mrs, B. That may have some sensible effect; but when 











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m r 


m 


* 
























. 




i 

















































Pl..\ II. XVI 






































































ON OPTICS. 


185 


an object on the summit of a hill has a back ground of light 
sky, the contrast with the object makes its outline more dis¬ 
tinct. 

Caroline. I now feel welt satisfied, that we see opaque 
objects only by reflected rays; but i do nut understand how 
these rays show us the objects from which they proceed? 

Mrs. B. The rays of light enter at the pupil of the eye, 
and proceed to the retina, or optic nerve, which is situated 
at the back part of the eye-ball; and there they describe the 
figure, colour, and (excepting size) form a perfect represen¬ 
tation of the object from which they proceed. We shall 
again close the shutters, and admit the light through the 
small aperture, and you will see a picture on the wall, op¬ 
posite the aperture, similar to that which is delineated on 
the retina of the eye. 

Caroline. Oh, how wonderful! There is an exact pic¬ 
ture in miniature of the garden, the gardener at work, the 
trees blown about by the wind. The landscape would be 
perfect, if it were not reversed; the ground being above, and 
the sky beneath. 

Mrs. B. It is not enough to admire, you must under¬ 
stand this phenomenon, which is called a camera obscura, 
from the necessity of darkening the room, in order to exhi¬ 
bit it. 

This picture is produced by the rays of light reflected 
from the various objects in the garden, and which are ad¬ 
mitted through the hole in the window shutter. 

The rays from the glittering weathercock at the top of 
the alcove A, (plate XVI. fig. 1.) represent it in this spot a; 
for the weathercock being much higher than the aperture 
in the shutter, only a few of the rays, which are reflected 
by it in an obliquely descending direction, can find entrance 
there. The rays of light, you know, always move in straight 
lines; those, therefore, which enter the room in a descend¬ 
ing direction, will continue their course in the same direc¬ 
tion, and will consequently fall upon the lower part of the 
wall opposite the aperture, and represent the weathercock 
reversed in that spot, instead of erect in the uppermost part 
of the landscape. 


186 


ON OPTICS. 


Emily . And the rap of light from the steps (B) of the 
alcove, in entering the aperture, ascend, and will describe 
those steps in the highest instead of the lowest part of the 
landscape. 

Mrs. B. Observe, too, that the rays coming from the 
alcove, which is to our left, describe it on the wall to the 
right; while those which are reflected by the walnut-tree 
C D, to our right, delineate its figure in the picture to the 
left c d. Thus the rays, coming in different directions, and 
proceeding always in right lines, cross each other at their 
entrance through the aperture; those which come above pro¬ 
ceed below, those from the right go to the left, those from 
the left towards the right; thus every object is represented 
in the picture, as occupying a situation the very reverse of 
that which it does in nature. 

Caroline . Excepting the flower-pot E F, which, though 
its position is reversed, has not changed its situation in the 
landscape. 

Mrs. B . The flower-pot is directly in front of the aper¬ 
ture; so that its rays fall perpendicularly upon it, and con¬ 
sequently proceed perpendicularly to the wall, where they 
delineate the object directly behind the aperture. 

Emily . And is it thus that the picture of objects is paint¬ 
ed on the retina of the eye? 

Mrs. B. Precisely. The pupil of the eye, through which 
the rays of light enter, represents the aperture in the win¬ 
dow-shutter; and the image delineated on the retina, is ex¬ 
actly similar to the picture on the wall. 

Caroline. You do not mean to say, that we see only the 
representation of the object which is painted on the retina, 
and not the object itself? 

Mrs. B. If, by sight, you understand that sense by which 
the presence of objects is perceived by the mind, through 
the means of the eyes, we certainly see only the image of 
those objects painted on the retina. 

Caroline. This appears to me quite incredible. 

Mrs. B. The nerves are the only part of our frame ca¬ 
pable of sensation: they appear, therefore, to be the instru¬ 
ments which the mind employs in its perceptions; for a sen- 


ON OPTICS. 


187 


sation always conveys an idea to the mind. Now it is known, 
that our nerves can be affected only by contact; and for this 
reason the organs of sense can not act at a distance: for in¬ 
stance, we are capable of smelling only particles which are 
actually in contact with the nerves of the nose. We have 
already observed, that the odour of a flower consists in efflu¬ 
via, composed of very minute particles, which penetrate the 
nostrils, and strike upon the olfactory nerves, which instantly 
convey the idea of smell to the mind. 

Emily. And sound, though it is said to be heard at a 
distance, is, in fact, heard only when the vibrations of the 
air, which convey it to our ears, strike upon the auditory 
nerve. 

Caroline. There is no explanation required, to prove 
that the senses of feeling and of tasting are excited only by 
contact. 

Mrs. B. And I hope to convince you, that the sense of 
sight is so likewise. The nerves, which constitute the sense 
of sight, are not different in their nature from those of the 
other organs; they are merely instruments which convey 
ideas to the mind, and can be affected only on contact 
Now, since real objects can not be brought to touch the 
optic nerve, the image of them is conveyed thither by the 
rays of light proceeding from real objects which actually 
strike upon the optic nerve, and form that image which the 
mind perceives. 

Caroline. While I listen to your reasoning, I feel con¬ 
vinced; but when I look upon the objects around, and think 
that I do not see them, but merely their image painted in 
my eyes, my belief is again staggered. I can not reconcile 
myself to the idea, that f do not really see this book which I 
hold in my hand, nor the words which I read in it. 

Mrs. B. Did it ever occur to you as extraordinary, that 
you never beheld your own face? 

Caroline. No; because I so frequently see an exact re¬ 
presentation of it in the looking-glass, 

Mrs. B. You see a far more exact representation of ob¬ 
jects on the retina of your eye: it is a much more perfect 
mirror than any made by art. 


188 


ON OPTICS. 


Entity. But is it possible, that the extensive landscape, 
which 1 now behold front the window, should be represent¬ 
ed on so small a space as the retina of the eye? 

Mrs. B. It would be impossible for art to paint so small 
and distinct a miniature; but nature works with a surer 
hand, and a more delicate pencil. That power, which forms 
the feathers of the butterfly, and the flowerets of the daisy, 
cau alone pourtray so admirable and perfect a miniature as 
that which is represented on the retina of the eye. 

Caroline. But, Mrs. B., if we see only the image of ob¬ 
jects, why do we not see them reversed, as you showed us 
they were in the camera obscura? Is not that a strong argu¬ 
ment against your theory? 

Mrs. B. Not an unanswerable one, I hope. The image 
on the retina, it is true, is reversed, like that in the camera 
obscura; as the rays, unless from a very small object, inter¬ 
sect each other on entering the pupil, in the same manner 
as they do on entering the camera obscura. The scene, 
however, does not excite the idea of being inverted, because 
we always see an object in the direction of the rays which 
it sends to us. 

Emily. I confess I do not understand that. 

Mrs. B. It is, I think, a difficult point to explain clearly. 
A ray which comes from the upper part of an object, de¬ 
scribes the image on the lower part of the retina; but, ex¬ 
perience having taught us, that the direction of that ray is 
from above, we consider that part of the object it represents 
as uppermost. The rays proceeding from the lower part of 
an object fall upon the upper part of the retina; but as we 
know their direction to be from below, we see that part of 
the object they describe as the lowest. 

Caroline. When I want to see an object above me, I 
look up; when an object below me, I look down. Does not 
this prove that I see the objects themselves? for if I beheld 
only the image, there would be no necessity for looking up 
or down, according as the object was higher or lower than 
myself. 

Mrs . B . I beg your pardon. When you look up to an 
elevated object, it is in order that the rays reflected from it 


ON OPTICS. 


189 


should fall upon the retina of your eyes; but the very cir¬ 
cumstance of directing your eyes upwards, convinces you 
that the object is elevated, and teaches you to consider as 
uppermost the image it forms on the retina, though it is, in 
fact, represented in the lowest part of it. When you look 
down upon an object, you drawjour conclusion from a simi¬ 
lar reasoning; it is thus that we see all objects in the direc¬ 
tion of the rays which reach our eyes. T> 

But I have a further proof in favour of wh r ' 1 have ad¬ 
vanced, which I hope will remove your remaining doubts: I 
shall, however, defer it till our next meeting, as the lesson 
has been sufficiently long to-day. 


17 


CONVERSATION XV. 



OPTIC S— continued . 

ON THE ANGLE OF VISION, AND THE REFLECTION OF MIRRORS. 

ANGLE OP VISION.—REFLECTION OF PLAIN MIRRORS.—REFLECTION OF 
CONVEX MIRRORS.-REFLECTION OF CONCAVE MIRRORS. 

CAROLINE. 

Well, Mrs. B., I am very impatient to bear what fur¬ 
ther proofs you have to offer in support of your theory. You 
must allow that it was rather provoking to dismiss us as you 
did at our last meeting. 

Mrs. B . You press so hard upon me with your objec¬ 
tions, that you must give me time to recruit my forces. 

Can you tell me, Caroline, why objects at a distance ap¬ 
pear smaller than they really are? 

Caroline. I know no other reason than their distance. 

Mrs. B. I do not think I have more cause to be satis¬ 
fied with your reasons, than you appear to be with mine. 
We must refer again to the camera obscura to account for 
this circumstance; and you will find, that the different ap¬ 
parent dimensions of objects at different distances, proceed 
from oijr seeing, not the objects themselves, but merely their 
image on the retina. Fig. 1. plate XVII. represents a row 
of trees, as viewed in the camera obscura. I have expressed 
the direction of the rays, from the objects to the image, by 
lines. Now, observe, the ray which comes from the top 




Pl.atk.xvu . 





















































•* 









\ 










ON TIIE ANGLE OF VISION. 


191 


of the nearest tree, and that which comes from the foot of 
the same tree, meet at the aperture, forming an angle of 
about twenty-five degrees; this is called the angle of vision, 
under which we seethe tree. These rays cross each other 
at the aperture, forming equal angles on each side of it, and 
represent the tree inverted in the camera obscura. The 
degrees of the image are considerably smaller than those of 
the object, but the proportions are perfectly preserved. 

Now let us notice the upper and lower ray, from the most 
distant tree; they form an angle of not more than twelve or 
fifteen degrees, and an image of proportional dimensions. 
Thus, two objects of the same size, as the two trees of the 
avenue, form figures of different sizes in the camera obscura, 
according to their distance; or, in other words, according to 
the angle of vision under which they are seen. Do you 
understand this? 

Caroline. Perfectly. 

Mrs. B. Then you have only to suppose that the repre¬ 
sentation in the camera obscura is similar to that on the 
retina. 

Now since objects of the same magnitudes appear to be 
of different dimensions, when at different distances from us, 
let me ask you, which it is that you see; the real objects, 
which we know do not vary in size, or the images,. which 
we know do vary according to the angle of vision unde? 
which we see them? 

Caroline . I must confess, that reason is in favour of the 
latter. But does that chair at the further end of the room 
form an image on my retina much smaller than this which 
is close to me? they appear exactly of the same size. 

Mrs. B. I assure you they do not.‘ The experience we 
acquire by the sense of touch corrects the errors of our sight 
with regard to objects within our reach. You are so per- 
ferfectly convinced of the real size of objects which you can 
handle, that you do not attend to their apparent difference. 

Does that house appear to you much smaller than when 
you are close to it. 

Caroline. No, because it is very near us. 

Mrs. B . And yet you can see the whole of it through 


192 


ON THE ANGLE OP VISION. 


one of the windows of this room. The image of the house 
on your retina must, therefore, be smaller than that of the 
window through which you see it. It is your knowledge of 
the real size of the house which prevents your attending to 
its apparent magnitude. If you were accustomed to draw 
from nature, you would be fully aware of this difference. 

Emily, And pray, what is the reason that, when we 
look up an avenue, the trees not only appear smaller as they 
are more distant, but seem gradually to approach each other 
till they meet in a point? 

Mrs. B, Not only the trees, but the road which sepa¬ 
rates the two row's, forms a smaller visual angle, in propor¬ 
tion as it is more distant from us; therefore the width of the 
road gradually diminishes as well as the size of the trees, 
till at length the road apparently terminates in a point, at 
which the trees seem to meet. 

But this effect of the angle of vision will be more fully 
illustrated by a little model of an avenue, which 1 have made 
for that purpose. It consists of six trees, leading to a hexa¬ 
gonal temple, and viewed by an eye, on the retina of which 
the picture of the objects is delineated. y 

I beg that yo,u will not criticise the proportions, for though 
the eye is represented the size of life, while the trees are 
not more than three inches high, the disproportion does not 
affect the principle, which the model is intended to elucidate. 

Emily. The threads which pass from the objects through 
the pupil of the eye to the retina, are, I suppose, to represent 
the rays of light which convey the image of the objects to 
the retina? 

Mrs. B . Yes. I have been obliged to limit the rays to 
a very small number, in order to avoid confusion; there are, 
you see, only two from each tree. 

Caroline. But as one is from the summit, and the other 
from the foot of the tree, they exemplify the different angles 
under which we see objects at different distances, better 
than if there were more. 

Mrs. B . There are seven rays proceeding from the 
temple, one from the summit, and two from each of the 
angles that are visible to the eye, as it is situated; from these 


ON THE ANGLE OF VISION. 


193 


you may form a just idea of the difference of the angle of 
vision of objects viewed obliquely, or in front; for though 
the six sides of the temple are of equal dimensions, that 
which is opposite to the eye is seen under a much larger 
angle, than those which are viewed obliquely. It is on this 
principle that the laws of perspective are founded. 

Emily. I am very glad to know that, for I have lately 
begun to learn perspective, which appeared to me a very 
dry study; but now that I am acquainted with the principles 
on which it is founded, I shall find it much more interesting. 

Caroline. In drawing a view from nature, then, we do 
not copy the real objects, but the image they form on the 
retina of our eyes? 

Mrs, B. Certainly. In sculpture, we copy nature as 
she really exists; in painting, we represent her as she ap¬ 
pears to us. It was on this account that I found it difficult 
to explain by a drawing the effects of the angle of vision, 
and was under the necessity of constructing a model for that 
purpose. 

Emily . I hope you will allow us to keep this model 
some time, in order to study it more completely, for a great 
deal may be learned from it; it illustrates the nature of the 
angle of vision, the apparent diminution of distant objects, 
and the inversion of the image on the retina. But pray, 
why are the threads that represent the rays of light, colour¬ 
ed, the same as the objects from which they proceed? 

Mrs. B. That is a question which you must excuse my 
answering at present, but I promise to explain it to you in 
due time. 

1 consent very willingly to your keeping the model on 
condition that you will make an imitation of it, on the same 
principle, but representing Jiff rent objects. 

We must now conclude the observations that remain to 
be made on the angle of vision. 

If an object, with an ordinary degree of illumination, 
does not subtend an angle of more than two seconds of a de¬ 
gree, it is invisible. There are consequently two cases in 
which objects may be invisible, either if they are too small, 
17 * 


194 


ON THE ANGLE OF VISION. 


or so distant as to form an angle less than two seconds of a 
degree. 

In like manner, if the velocity of a body does not exceed 
20 degrees in an hour, its motion is imperceptible. 

Caroline. A very rapid motion may then be impercepti¬ 
ble, provided the distance of the moving body is sufficiently 
great. 

Mrs. B. Undoubtedly; for the greater its distance, the 
smaller will be the angle under which its motion will ap¬ 
pear to the eye. It is for this reason that the motion of the 
celestial bodies is invisible, notwithstanding their immense 
velocity. 

Emily. I am surprised that so great a velocity as 20 de¬ 
grees au hour should be invisible. 

Mrs. B. The real velocity depends altogether on the 
space comprehended in each degree; and this space depends 
on the distance of the object, and the obliquity of its path. 
Observe, likewise, that we can not judge of the velocity of 
a body in motion unless we know its distance; for supposing 
two men to set off at the same moment from A and B, (fig. 
2.) to walk each to the end of their respective lines C and 
D; if they perform their walk in the same space of time, 
they must have proceeded at a very different rate, and yet to 
an eye situated at E, they will appear to have moved with 
equal velocity: because they will both have gone through an 
equal number of degrees, though over a very unequal length 
of ground. Sight is an extremely useful sense no doubt, 
but it can not always be relied on, it deceives us both in 
regard to the size and the distance of objects; indeed our 
senses would be very liable to lead us into error, if experi¬ 
ence did not set us right. 

Emily. Between the two, I think that we contrive to 
•acquire a tolerably accurate idea of objects. 

Mrs. B. At least sufficiently so tor the general purposes 
of life. To convince you how requisite experience is to 
correct the errors of sight, I shall relate to you the case of 
a young man who was blind from bis infancy, and who re¬ 
covered his sight at the age of fourteen, by the operation of 
couching. At first, he had no idea either of the size or dis- 


ON THE ANGLE OF VISION, 


195 


tance of objects, but imagined that every thing he saw touch¬ 
ed his eyes; and it was not till after having repeatedly felt 
them, and walked from one object to another, that he ac¬ 
quired an idea of their respective dimensions, their relative 
situations, and their distances. 

Caroline. The idea that objects touched his eyes, is 
however not so absurd, as it at first appears; for if we con¬ 
sider that we see only the image of objects, this image ac¬ 
tually touches our eyes. 

Mrs . B. That is doubtless the reason of the opinion he 
formed, before the sense of touch had corrected his judgment. 

Caroline. But since an image must be formed on the 
retina of each of our eyes, why do we not see objects double? 

J\lrs. B. The action of the rays on the optic nerve of 
each eye is so perfectly similar, that they produce but a single 
sensation, the mind therefore receives the same idea, from 
the retina of both eyes, and conceives the object to be single. 

Caroline. This is difficult to comprehend, and 1 should 
think, can be but conjectural. 

Mrs. B. I can easily convince you, that you have a 
distinct image of an object formed on the retina of each eye. 
Look at the bell rope, and tell me do you see it to the right 
or the left of the pole of the fire-skreen? 

Caroline . A little to the right of it. 

Mrs. B. Then shut your right eye, and you will see it 
to the left of the pole. 

Caroline. That is true indeed! 

Mrs. B. There are evidently two representations of the 
bell-rope in different situations, which must be owing to an 
image of it being formed on both eyes; if the action of the 
rays therefore on each retina were not so perfectly similar 
as to produce but one sensation, we should see double, and 
we find that to be the case with many persons who are af¬ 
flicted with a disease in one eye, which prevents the rays of 
light from affecting it in the same manner as the other. 

Emily. Pray, Mrs. B., when we see the image of an 
object in a looking-glass, why is it not inverted as in the 
camera obscura, and on the retina of the eye? 

Mrs . B. Because the rays do not enter the mirror by a 


196 


REFLECTION OF MIRRORS. 


small aperture, and cross each other, as they do at the orifice 
of a camera obscura, or the pupil of the eye. 

When you view yourself in a mirror, the rays from your 
eyes fall perpendicularly upon it, and arc reflected in the 
same line; the image is therefore described behind the 
glass, and is situated in the same manner as the object be¬ 
fore it. 

Emily . Yes, I see that it is; but the looking-glass is not 
nearly so tall as I am, how is it therefore that I can see the 
whole of my figure in it? 

Mrs. B. it is not necessary that the mirror should be 
more than half vour height, in order that you may see the 
whole of your person in it, (fig. 3.) The ray of light AB 
from your eye, which falls perpendicularly on the mirror 
B 1), will be reflected back in the same line; but the ray 
from your feet will fall obliquely on the mirror, for it must 
ascend in order to reach it; it will therefore be reflected in 
the line A D: and since we view objects in the direction of 
the reflected rays, which reach the eye, and that the image 
appears at the same distance behind the mirror that the ob¬ 
ject is before it, we must continue the line A D to E, and 
the line C D to F, at the termination of which, the image 
will be represented. 

Emily. Then I do not understand why I should not see 
the whole of my person in a much smaller mirror, for a ray 
of light from my feet would always reach it, though more 
obliquely. 

Mrs. B. True; but the more obliquely the ray falls on 
the mirror, the more obliquely it will be reflected; the ray 
would therefore be reflected above your head, and you could 
not see it. This is shown by the dotted line (fig. 3.) 

Now stand a little to the right of the mirror, so that the 
rays of light from your figure may fall obliquely on it —— 

Emily. There is no image formed of me in the glass 
now. 

Mrs. B. I beg your pardon, there is; but you can not 
see it, because the incident rays tailing obliquely on the 
mirror will be reflected obliquely in the opposite direction, 
the angles of incidence and of reflection being equal. Caro* 


REFLECTION OF MIRRORS. 


197 


line, place yourself in the direction of the reflected rays, 
and tell me whether you do not see Emily’s image in the 
glass? 

Caroline. Let me consider—In order to look in the di¬ 
rection of the reflected rays, I must place myself as much 
to the left of the glass as Emily stands to the right of it.— 
Now I see her image, but it is not straight before me, but 
before her; and appears at the same distance behind the 
glass, as she is in front of it. 

Mrs. B. You must recollect, that tve always see objects 
in the direction of the last rays which reach our eyes. Figure 
4 represents an eye looking at the image of a vase, reflected 
by a mirror; it must see it in the direction of the ray A B, 
as that is the ray which brings the image to the eye: prolong 
the ray to C, and in that spot will the image appear. 

Caroline. I do not understand why a looking-glass re¬ 
flects the rays of light; for glass is a transparent body which 
should transmit them? 

Mrs. B. It is not the glass that reflects the rays which 
form the image you behold, but the mercury behind it. The 
glass acts chiefly as a transparent case, through which the 
rays find an easy passage. 

Caroline. Why then should not mirrors be made simply 
of mercury? 

Mrs. B. Because mercury is a fluid. By amalgamating 
it with tin-foil, it becomes of the consistence of paste, at¬ 
taches itself to the glass, and forms in fact a mercurial mir¬ 
ror, which would be much more perfect without its glass 
cover, for the purest glass is never perfectly transparent; 
some of the rays therefore are lost during their passage 
through it, by being either absorbed, or irregularly re¬ 
flected. 

This imperfection of glass mirrors has introduced the use 
of metallic mirrors, for optical purposes. 

Emily. But since all opaque bodies reflect the rays of 
light, I do not understand why they are not all mirrors? 

Caroline. A curious idea indeed, sister; it would be 
very gratifying to see oneself in every object at which one 
looked. 


198 


REFLECTION OF MIRRORS. 


Mrs. B. It is very true that all opaque objects reflect 
light; but the surface of bodies in general is so rough and 
uneven, that their reflection is extremely irregular, which 
prevents the rays from forming an image on the retina. 
This you will be able to understand better, when I shall 
explain to you the nature of vision, and the structure of the 
eye. 

You may easily conceive the variety of directions in which 
rays would be reflected by a nutmeg-grater, on account of 
the inequality of its surface, and the number of holes with 
which it is pierced. All solid bodies resemble the nutmeg- 
grater in these respects, more or less; and it is only those 
which are susceptible of receiving a polish, that can be made 
to reflect the rays with regularity. As hard bodies are of 
the closest texture, the least porous, and capable of taking 
the highest polish, they make the best mirrors; none there¬ 
fore are so well calculated for this purpose as metals. 

Caroline . But the property of regular reflection is not 
confined to this class of bodies; for I have often seen myself 
in a highly polished mahogany table. 

Mrs. B. Certainly; but as that substance is less durable, 
and its reflection less perfect, than that of metals, I believe 
it would seldom be chosen for the purpose of a mirror. 

There are three kinds of mirrors used in optics; the plain 
or flat, which are the common mirrors we have just men¬ 
tioned; convex mirrors, and concave mirrors. The reflec¬ 
tion of the two latter is very different from that of the former. 
The plain mirror, we have seen, does not alter the direction 
of the reflected rays, and forms an image behind the glass 
exactly similar to the object before it. A convex mirror has 
the peculiar property of'making the reflected rays diverge, 
by which means it diminishes the image; and a concave 
mirror makes the rays converge, and under certain circum¬ 
stances, magnifies the image. 

Emily. We have a convex mirror in the drawing-room, 
which forms a beautiful miniature picture of the objects in 
the room; and I have often amused myself with looking at 
my magnified face in a concave mirror.' But I hope you will 


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REFLECTION OF CONVEX MIRRORS. 199 

explain to us why the one enlarges, while the other dimin¬ 
ishes the objects it reflects. 

Mrs. B. Let us begin by examining the reflection of a 
convex mirror. This is formed of a portion of the exterior 
surface of a sphere. When several parallel rays fall upon 
it, that ray only which, if prolonged, would pass through the 
centre or axis of the mirror, is perpendicular to it. In order 
to avoid confusion, I have, in fig. 1. plate XVIII., drawn 
only three parallel lines, A B, C D, E F, to represent rays 
falling on the convex mirror M N; the middle ray, you will 
observe, is perpendicular to the mirror, the others fall on it 
obliquely. 

Caroline. As the three rays are parallel, why are they 
not all perpendicular to the mirror? 

Mrs. B. They would be so to a flat mirror; but as this 
is spherical, no ray can fall perpendicularly upon it which 
is not directed towards the centre of the sphere. 

Emily. Just as a weight falls perpendicularly to the earth 
when gravity attracts it towards the centre. 

Mrs. B. In order, therefore, that rays may fall perpen¬ 
dicularly to the mirror at B and F, the rays must be in the 
direction of the dotted lines, which, you may observe, meet 
at the centre 0 of the sphere, of which the mirror forms a 
portion. 

Now can you tell me in what direction the three rays, 

A B, C t>, E F, will be reflected? _ 

Emily. Yes, I think so: the middle ray falling perpen¬ 
dicularly on the mirror, will be reflected in the same line: 
the two others falling obliquely, will be reflected obliquely 
to GH; for the dotted lines you have drawn are perpendi¬ 
culars, which divide their angles of incidence and reflec¬ 
tion. 

Mrs. B. Extremely well, Emily: and since we see ob- j 
jects in the direction of the reflected ray, we shall see the 
image L, which is the point at which the reflected rays, if 
continued through the mirror, would unite and form an 
image. This point is equally distant, from the surface and 
centre of the sphere, and is called the imaginary focus of 
the mirror. 


200 REFLECTION OF CONVEX MIRRORS. 

Caroline. Pray, what is the meaning of focus? 

Mrs. B. A point at which converging rays unite. And 
it is in this case called an imaginary focus; because the rays 
do not really unite at that point, but only appear to do so: 
for the rays do not pass through the mirror, since they are 
reflected by it. 

Emily. I do not yet understand why an object appears 
smaller when viewed in a convex mirror. 

Mrs. B. It is owing to the divergence of the reflected 
rays. You have seen that a convex mirror converts, by re¬ 
flection, parallel rays into divergent rays; rays that fall upon 
the mirror divergent, are rendered still more so by reflec¬ 
tion, and convergent rays are reflected either parallel, or 
less convergent. If then an object be placed before any 
part of a convex mirror, as the vase A B, fig. 2. for instance, 
the two rays from its extremities, falling convergent on the 
mirror, will be reflected less convergent, and will not come 
to a focus till they arrive at C; then an eye placed in the 
direction of the reflected rays, will see the image formed in 
(or rather behind) the mirror at a b. 

Caroline. But the reflected rays do not appear to me to 
converge less than the incident rays. I should have sup¬ 
posed that, on the contrary, they converged more, since they 
meet in a point? 

Mrs. B. They would unite sooner than they actually do, 
if they were not less, convergent than the incident rays: for 
observe, that if the incident rays, instead of being reflected 
by the mirror, continued their course in their original direc¬ 
tion, they would come to a focus at D, which is considerably 
nearer to the mirror than at C; the image is therelore seen 
under a smaller angle than the object; and the more distant 
the latter is from the mirror, the less is the image reflected 
by it. 

You will now easily understand the nature of the reflec¬ 
tion of concave mirrors. These are formed of a portion of 
the internal surface of a hollow sphere, and their peculiar 
property is to converge the rays of light. 

Can you discover, Caroline, in what direction the three 


REFLECTION OF CONCAVE MIRRORS. 


20 i 


parallel rays, A B, CD, E P, which fall on the concave 
mirror MN, (fig. 3.) are reflected? 

Caroline. I believe I can. The middle ray is sent back 
in the same line, as it is in the direction of the axis of the 
mirror; and the two others will be reflected obliquely, as 
they fall obliquely on the mirror. I must now draw two 
dotted lines perpendicular to their points of incidence, which 
will divide their angles of incidence and reflection; and in 
order that those angles may be equal, the two oblique rays 
must be reflected to L, where they will unite with the mid¬ 
dle ray. 

Mrs. B. Very well explained. Thus you see, that when 
any number of parallel rays fall on a concave mirror, they 
are all reflected to a focus: for in proportion as the ravs are 
more distant from the axis of the mirror, they fall more ob¬ 
liquely upon it, and are more obliquely reflected; in conse¬ 
quence of which they come to a focus in the direction of 
the axis of the mirror, at a point equally distant from the 
centre and the surface of the sphere, and this point is not an 
imaginary focus, as happens with the convex mirror, but is 
the true focus at which the rays unite. 

Emily. Can a mirror form more than one focus by re¬ 
flecting rays? 

Mrs. B. Yes. If rays fall convergent on a concave mir¬ 
ror, (fig. 4.) they are sooner brought to a focus, L. than 
parallel rays; their focus is therefore nearer to the mirror 
MN. Divergent rays are brought to a more distant focus 
than parallel rays, as in figure 5, where the focus is at L; 
but the true focus of mirrors, either convex or concave, is 
that of parallel rays, which is equally distant from the cen¬ 
tre, and the surface of the sphere. 

I shall now show you the reflection of real rays of liffht, 

v t v o • 

by a metallic concave mirror. This is one made of polish¬ 
ed tin, which I expose to the sun, and as it shines bright, 
we shall be able to collect the rays into a very brilliant focus. 
I hold a piece cf paper where I imagine the focus to be 
situated; you may see by the vivid spot of light on the paper, 
how much the rays converge: but it is not yet exactly in the 
focus; as I approach the paper to that point, observe how 


202 


REFLECTION OF CONCAVE MIRRORS. 


the brightness of the spot of light increases, while its size 
diminishes. 

Caroline. That must be occasioned by the rays becom¬ 
ing closer together. I think you hold the paper just in the 
focus now, the light is so small and dazzling—Oh, Mrs. B., 
the paper has taken fire! 

Mrs. B. The rays of light can not be concentrated, 
without, at the same time, accumulating a proportional 
quantity of heat: hence concave mirrors have obtained the 
name of burning-mirrors. 

Emily. I have often heard of the surprising effects of 
burning-mirrors, and I am quite delighted to understand 
their nature. 

Caroline. It can not be the true focus of the mirror at 
which the rays of the sun unite, for as they proceed from a 
point, they must fall divergent upon the mirror. 

Mrs. B. Strictly speaking, they certainly do. But when 
rays come from such an immense distance as the sun, their 
divergence is so trifling, as to be imperceptible; and they 
may be considered as parallel: their point of union is, there¬ 
fore, the true focus of the mirror, and there the image of the 
object is represented. 

Now that I have removed the mirror out of the influence 
of the sun’s rays, if I place a burning taper in the focus, 
how will its light be reflected? (Fig. 6.) 

Caroline. That, I confess, I can not say. 

Mrs. B. The ray which falls in the direction of the 
axis of the mirror, is reflected back in the same line; but 
let us draw two other rays from the focus, falling on the 
mirror at B and F; the dotted lines are perpendicular to 
those points, and the two rays will therefore be reflected to 
A and E. 

Caroline. Oh, now I understand it clearly. The rays 
which proceed from a light placed in the focus of a concave 
mirror fall divergent upon it, and are reflected parallel. It 
is exactly the reverse of the former experiment, in which 
the sun’s rays fell parallel on the mirror, and were reflected 
to a focus. 

Mrs.B. Yes: when the incident rays are parallel, the 


THE REFLECTION OF MIRRORS. 


203 


reflected rays converge to a focus; when, on the contrary, 
the incident rays proceed from the focus, they are reflected 
parallel. This is an important law of optics, and since you 
are now acquainted with the principles on which it is found¬ 
ed, I hope that you will not forget it. 

Caroline . I am sure that we shall not. But, Mrs. B., 
you said that the image was formed in the focus of a concave 
mirror; yet I have frequently seen glass concave mirrors, 
where the object has been represented within the mirror, in 
the same manner as in a convex mirror. 

Mrs. B. That is the case only, when the object is placed 
between the mirror and its focus; the image then appears 
magnified behind, or, as you call it, within the mirror. 

Caroline . I do not understand why the image should be 
larger than the object. 

Mrs . B. It proceeds from the convergent property of the 
concave mirror. If an object, A B, (fig. 7.) be placed be¬ 
tween the mirror and its focus, the rays from its extremities 
fall divergent on the mirror, and on being reflected, become 
less divergent, as if they proceeded from C: to an eye placed 
in that situation the image will appear magnified behind the 
mirror at a b } since it is seen under a larger angle than the 
object. 

You now, I hope, understand the reflection of light by 
opaque bodies. At our next meeting, we shall enter upon 
another property of light, no less interesting, which is Galled 
refraction. 


CONVERSATION XVI. 


ON REFRACTION AND COLOURS. 


TRANSMISSION OF LIGHT BT TRANSPARENT BODIES.—REFRACTION.-RE¬ 
FRACTION THE ATMOSPHERE-REFRACTION OF A LENS.-REFRACTION 

OF THE PRISM.-OF THE COLOURS OF RA ES OF LIGHT.—OF THE COLOUH8 

OF BODIES. 


MRS. B. 

The refraotion of light will furnish the subject of to day’s 
lesson. 

Caroline. That is a property of which I have not the 
faintest idea. 

Mrs . B. It is the effect which transparent mediums 
produce on light in its passage through them. Opaque bo¬ 
dies, you know, reflect the rays, and transparent bodies 
transmit them; but it is found, that if a ray, in passing from 
one medium into another of different density, fall obliquely, 
it is turned out of its course. 

Caroline. It must then be acted on by some new power, 
otherwise it would not deviate from its first direction. 

Mrs. B. The power which causes the deviation of the 
ray appears to be the attraction of the denser medium. Let 
us suppose the two mediums to be air and water; if a ray of 
light passes from air into water, it is more strongly attracted 
by the latter on account of its superior density. 

Emily. In what direction does the water attract the ray ? 

Mrs. B. It must attract it perpendicularly towards it, 
in the same manner as gravity acts on bodies. 


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205 


THE REFRACTION OF LIGHT. 

If then a ray, A B, (fig. 1. plate XIX.) fall perpendicu¬ 
larly on water, the attraction of the water acts in the same 
direction as the course of the ray: it will not therefore cause 
a deviation, and the ray will proceed straight on to E, But 
if it fall obliquely, as the ray C B, the water will attract it 
out of its course. Let us suppose the ray to have approach¬ 
ed the surface of a denser medium, and that it there begins 
to be affected by its attraction; this attraction, if not coun¬ 
teracted by some other power, would draw it perpendicu¬ 
larly to the water, atB; but it is also impelled by its projec¬ 
tile force, which the attraction of the denser medium can 
not overcome; the ray, therefore, acted on by both these pow¬ 
ers, moves in a direction between them, and instead of pur¬ 
suing its original course to D, or being implicitly guided bv 
the water to E, proceeds towards F, so that the ray appears 
bent or broken. 

Caroline . I understand that very well; and is not this 
the reason that oars appear bent in water? 

Mrs . B. It is owing to the refraction of the rays reflect¬ 
ed by the oar; but this is in passing from a dense to a rare 
medium, for you know that the rays, by means of which you 
see the oar, pass from water into air. 

Emily. But I do not understand why a refraction takes 
place when a ray passes from a dense into a rare medium; 
I should suppose that it would be rather less, than more, 
attracted by the latter. 

Mrs. B. And it is precisely on that account that the ray 
is refracted. C B, fig. 2. represents a ray passing obliquely 
from the glass into water: glass being the denser medium, 
the ray will be more strongly attracted by that which it leaves 
than by that which it enters. The attraction of the glass 
acts in the direction A B, while the impulse of projection 
would carry the ray to F; it moves, therefore, between these 
directions towards D. 

Emily. So that a contrary refraction takes place when 
a ray passes from a dense into a rare medium. 

Caroline. ' But does not the attraction of the denser 
medium affect the ray before it touches it? 

Mrs. B. The distance at which the attraction of the 
18 * 




206 


THE REFRACTION OF LIGHT. 


denser medium acts upon a ray is so small as to be insen¬ 
sible; it appears therefore to be refracted only at the point 
at which it passes from one medium to the other. 

Now that you understand the principle of refraction, I 
will show you the refraction of a real ray of light. Do you 
see the flower painted at the bottom of the inside of this 
tea-cup? (Fig. 3.) 

Emily. Yes.—But now you have moved it just out of 
sight, the rim of the cup hides it. 

Mrs. B. Do not stir. I will fill the cup with water, 
and you will see the flower again. 

Emily. I do indeed! Let me try to explain this: when 
you draw the cup from me so as to conceal the flower, the 
rays reflected by it no longer meet my eyes, but were direct¬ 
ed above them; but now that you have filled the cup with 
water, they are refracted by the attraction of the water, and 
bent downwards, so as again to enter my eyes. 

Mrs. B. You have explained it perfectly: fig. 3. will 
help to imprint it on your memory. You must observe that 
when the flower becomes visible by the refraction of the ray, 
you do not see it in the situation which it really occupies, 
but an image of the flower higher in the cup; for as objects 
always appear to be situated in the direction of the rays 
which enter the eye, the flower will be seen in the direction 
of the reflected ray B. 

Emily. Then, when w r e see the bottom of a clear stream 
of water, the rays which it reflects being refracted in their 
passage from the water into the air, will make the bottom 
appear higher than it really is. 

Mrs. B. And the water will consequently appear more 
shallow. Accidents have frequently been occasioned by this 
circumstance; and boys who are in the habit of bathing 
should bo cautioned not to trust to the apparent shallowness 
of water, as it will always prove deeper than it appears; 
unless,'* indeed, they view it from a boat on the water, which 
will enable them to look perpendicularly upon it; when the 
rays from the bottom passing perpendicularly, no refraction 
will take place. 

The refraction of light prevents our seeing the heavenly 


THE REFRACTION OF LIGHT. 


207 


bodies in their real situation: the light they send to us being 
refracted in passing into the atmosphere, we see the sun and 
stars in the direction of the refracted ray; as described in 
fig. 4, plate XIX., the dotted line represents the extent of 
the atmosphere, above a portion of the earth, E B E: a raj 
of light corning from the sun S falls obliquely on it at A, 
and is refracted to B; then, since we see the object in the 
direction of the refracted ray, a spectator at B will see an 
image of the sun at C, instead of the real object at S. 

Emily. But if the sun were immediately over our heads, 
its rays falling perpendicularly on the atmosphere would not 
be refracted, and we should then see the real sun, in its 
true situation. 

Mrs. B. You must recollect that the sun, is vertical 
only to the inhabitants of the torrid zone; its rays, therefore, 
are always refracted in these climates. There is also an¬ 
other obstacle to our seeing the heavenly bodies in their 
real situations: light, though it moves with extreme velocity, 
is about eight minutes and a half in its passage from the 
sun to the earth; therefore, when the rays reach us, the sun 
must have quitted the. spot he occupied on their departure; 
yet we see him in the direction of those rays, and conse¬ 
quently in a situation which he had abandoned eight minutes 
and a half before. 

Emily. When you speak of the sun’s motion, you mean, 
I suppose, his apparent motion, produced by the diurnal 
motion of the earth? 

Mrs . B. No doubt; the effect being the same, whether 
it is our earth, or the heavenly bodies which move: it is 
more easy to represent things as they appear to be, than as 
they really are. 

Caroline. During the morning, then, when the sun is 
rising towards the meridian, we must (from the length of 
time the light is in reaching us) see an image of the sun 
below that spot which it really occupies. 

Emily. But the refraction of the atmosphere counter¬ 
acting this effect, we may perhaps, between the two, see 
the sun in its real situation 

Caroline . And in the afternoon, when the sun is sinking 


208 


THE REFRACTION OF LIGHT. 


in the west, refraction and the length of time which the light 
is in reaching the earth, will conspire to render the image 
of the sun higher than it really is. 

Mrs. B. The refraction of the sun’s rays by the atmos¬ 
phere prolongs our days, as it occasions our seeing an image 
of the sun, both before he rises and after he sets; for below 
the horizon, he still shines upon the atmosphere, and his 
rays are thence refracted to the earth. So likewise we see 
an image of the sun before he rises, the rays that previously 
fall upon the atmosphere being reflected to the earth. 

Caroline. On the other hand, we must recollect that 
light is eight minutes and a half on its journey; so that, by 
the time it reaches the earth, the sun may perhaps be risen 
above the horizon. 

Emily. Pray do not glass-windows refract the light? 

Mrs. B. They do; but this refraction is not perceptible, 
because, in passing through a pane of glass, the rays suffer 
two refractions, which being in contrary directions, produce 
the same effect as if no refraction had taken place. 

Emily. I do not understand that. 

Mrs . B. Fig. 5. plate XIX. will make it clear to you: 
A A represents a thick pane of glass seen edgeways. When 
the ray B approaches the glass at C, it is refracted by it; 
and instead of continuing its course in the same direction, 
as the dotted line describes, it passes through the pane to 
D; at that point returning into the air it is again refracted 
by the glass, but in a contrary direction to the first refrac¬ 
tion, and in consequence proceeds to E. Now you must ob¬ 
serve that the ray B C and the ray D E being parallel, the 
light does not appear to have suffered any refraction. 

Emily. So that the effect which takes place on the ray 
entering the glass, is undone on its quitting it. Or, to ex¬ 
press myself more scientifically, when a ray of light passes 
from one medium into another, and through that into the 
first again, the two refractions being equal and in opposite 
directions, no sensible effect is produced. 

Mrs. B. This is the case when the two surfaces of the 
refracting medium are parallel to each other; if they are not, 


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ON REFRACTION AND COLOURS. 209 

the two refractions may be made in the same direction, as 
I shall show you. 

When parallel rays (fig. 6.) fall on a piece of glass hav¬ 
ing a double convex surface, and which is called a Lews, that 
only which falls in the direction of the axis of the lens is 
perpendicular to the surface; the other rays falling obliquely 
are refracted towards the axis, and will meet at a point be¬ 
yond the lens, called its focus. 

Of the three rays, ABC, which fall on the lens D E, the 
rays A and C are refracted in their passage through it, to «, 
and c, and on quitting the lens they undergo a second re¬ 
fraction in the same direction which unites them with the 
ray B, at the focus F. 

Emily. And what is the distance of the focus from the 
surface of the lens? 

Mrs. B. The focal distance depends both upon the form 
of the lens, and of the refractive power of the substance of 
which it is made: in a glass lens, both sides of which are 
equally convex, the focus is situated nearly at the centre of 
the sphere of which the surface of the lens forms a portion; 
it is at the distance, therefore, of the radius of the sphere. 

There are lenses of various forms, as you will find de¬ 
scribed in fig. 1. plate XX. The property of those which 
have a convex surface is to collect the rays of light to a fo¬ 
cus; and of those which have a concave surface, on the 
contrary, to disperse them. For the rays A C falling on 
the concave lens X Y, (fig, 7. plate XIX.) instead of con¬ 
verging towards the ray B, which falls on the axis of the 
lens, will each be attracted towards the thick edges of the 
Jens, both on entering and quitting it, and will, therefore, 
by the first refraction, be made to diverge to a, c, and by 
the second to d , e. 

Caroline. And lenses which have one side flat and the 
other convex or concave, as A and B, fig. 1. plate XX., are, 
I suppose, less powerful in their refractions? 

Mrs. B. Yes; they are called plano-convex, and plano¬ 
concave lenses: the focus of the former is at the distance of 
the diameter of a sphere, of which the convex surface of the 
lens forms a portion; as represented in fig. 2. plate XX, 


210 


ON REFRACTION AND COLOURS. 


The three parallel rays ABC, are brought to a focus by 
the plano-convex lens, X Y at F. 

I must now explain to you the refraction of a triangular 
piece of glass, called a prism. (Fig. 3.) 

Emily. The three sides of this glass are flat; it can not 
therefore bring the rays to a focus; nor do I suppose that its 
refraction will be similar to that of a flat pane of glass, be¬ 
cause it has not two sides parallel; I can not therefore con¬ 
jecture what effect the refraction of a prism can produce. 

Mrs. B. The refractions of the light, on entering and 
on quitting the prism, are both in the same direction, (Fig. 
3.) On entering the prism P, the ray A is refracted from 
B to C, and on quitting it from C to D. 

I will show you this in nature; but for this purpose it will 
be advisable to close the window-shutters, and admit, through 
the small aperture, a ray of light, which I shall refract by 
means of this prism. 

Caroline. Oh, what beautiful colours are represented on 
the opposite wall! There are all the colours of the rainbow, 
and with a brightness I never saw equalled. (Fig. 4. plate 
XX.) 

Emily . I have seen an effect, in some respects similar 
to this, produced by the rays of the sun shining upon glass 
lustres; but how is it possible that a piece of white glass 
can produce such a variety of brilliant colours? 

Mrs. B. The colours are not formed by the prism, but 
existed in the ray previous to its refraction. 

Caroline . Yet, before its refraction, it appeared perfectly 
white. 

Mrs B. The white rays of the sun are composed of 
coloured rays, which, when blended together, appear co¬ 
lourless or white. 

Sir Isaac Newton, to whom we are indebted for the most 
important discoveries respecting light and colours, was the 
first who divided a white ray of light, and found it to consist 
of an assemblage of coloured rays, which formed an image 
upon the wall, such as you now see exhibited, (fig. 4.) in 
which are displayed the following series of colours: red, 
orange, yellow, green, blue, indigo, and violet. 


ON REFRACTION AND COLOURS. 211 

Emily . But how does a prism separate these coloured 
rays? 

Mrs. B. By reflection. It appears that the coloured rays 
have different degrees of refrangibility; in passing through 
the prism, therefore, they take different directions according 
to their susceptibility of refraction. The violet rays deviate 
most from their original course; they appear at one of the 
ends of the spectrum A B: contiguous to the violet, are the 
blue rays, being those which have somewhat less refrangi¬ 
bility; then follow, in succession, the green, yellow, orange, 
and, lastly, the red, which are the least refrangible of the 
coloured rays. 

Caroline. I can not conceive how these colours, mixed 
together, can become white? 

Mrs. B. That I can not pretend to explain; but it is a 
fact that the union of these colours, in the proportions in 
which they appear in the spectrum, produce in us the idea 
of whiteness. If you paint a card in compartments with 
these seven colours, and whirl it rapidly on a pin, it will ap¬ 
pear white. 

But a more decisive proof of the composition of a white 
ray is afforded by reuniting these coloured rays, and form¬ 
ing with them a ray of white light. 

Caroline. If you can take a ray of white light to pieces, 
and put it together again, I sha*U be quite satisfied. 

Mrs. B. This can be done by letting the coloured rays, 
which have been separated by a prism, fall upon a lens, 
which will converge them to a focus; and if, when thus re¬ 
united, we find that they appear white as they did before 
refraciion, I hope that you will be convinced that the white 
rays are a compound of the several coloured rays. The 
prism P, you see, (fig. 5.) separates a ray of white light into 
seven coloured rays, and the lens L L brings them to a focus 
at F, where they again appear white. 

Caroline. You succeed to perfection: this is indeed a 
most interesting and conclusive experiment. 

Emily. Yet, Mrs. B., I can not help thinking, that there 
may perhaps be but three distinct colours in the spectrum, 
red, yellow, and blue; and that the four others may consist of 


212 


ON REFRACTION AND COLOURS. 


two of these colours blended together; for, in painting, we 
find that by mixing red and yellow, we produce orange; with 
different proportions of red and blue, w r e make violet or any 
shade of purple; and yellow and blue form green. Now it 
is very natural to suppose, that the refraction of a prism may 
not be so perfect as to separate the coloured rays of light 
completely, and that those which are contiguous in order of 
refrangibility may encroach on each other, and by mixing 
produce the intermediate colours, orange, green, violet, and 
indigo. 

Mrs. B. Your observation is, I believe, neither quite 
wrong, nor quite right. Dr. Wollaston, who has refracted 
light in a more accurate manner than had been previously 
done, by receiving a very narrow line of light on a prism, 
found that it formed a spectrum, consisting of rays of four 
colours only; but they were not exactly those you have named 
as primitive colours, for they consisted of red, green, blue, 
and violet. A very narrow line of yellow was visible, at 
the limit of the red and green, which Dr. Wollaston attri¬ 
buted to the overlapping of the edges of the red and green 
light. 

Caroline. But red and green mixed together, do not pro¬ 
duce yellow? 

Mrs. B. Not in painting; but it may be so in the primi¬ 
tive rays of the spectrum. Dr. Wollaston observed that, by 
increasing the breadth of the aperture by which the line of 
light was admitted, the space occupied by each coloured ray 
in the spectrum was augmented, in proportion as each por¬ 
tion encroached on the neighbouring colour and mixed with 
it; so that the intervention of orange and yellow, between 
the red and green, is owing, he supposes, to the mixture of 
these two colours, and the blue is blended on the one side 
with the green, and ou the other with the violet, forming 
the spectrum as it was originally observed by Sir Isaac 
Newton, and which I have just shown you. 

The rainbow, which exhibits a series of colours so analo¬ 
gous to those of the spectrum, is formed by the refraction of 
the sun’s rays in their passage through a shower of rain, 


ON REFRACTION AND COLOURS. 213 

every drop of which acts as a prism, in separating the co¬ 
loured rays as they pass through it. 

Emily. Pray, Mrs. B., can not the sun’s rays be collect¬ 
ed to a focus by a lens in the same manner as they are by a 
concave mirror? 

Mrs. B. No doubt the same effect is produced by the 
refraction of a lens as by the reflection of a concave mirror: 
in the first, the rays pass through the glass and converge to 
a focus behind it; in the latter, they are reflected from the 
mirror, and brought to a focus before it. A lens, when used 
for the purpose of collecting the sun’s rays, is called a burn¬ 
ing glass. The sun now shines very bright; if we let the 
rays fall on this lens you will perceive the focus. 

Emily. Oh yes: the point of union of the rays is very 
luminous. I will hold a piece of paper in the focus, and see 
if it will take fire. The spot of light is extremely brilliant, 
but the paper does not burn? 

Mrs. B. Try a piece of brown paper;—that you see 
takes fire almost immediately. 

Caroline. This is surprising; for the light appeared to 
shine more intensely on the white than on the brown paper. 

Mrs. B. The lens collects an equal number of rays to 
a focus, whether you hold the white or the brown paper 
there; but the white paper appears more luminous in the 
focus, because most of the rays, instead of entering into the 
paper, are reflected by it; and this is the reason that the pa¬ 
per is not burnt: whilst, on the contrary, the brown paper, 
which absorbs more light than it reflects, soon becomes heat¬ 
ed and takes fire. 

Caroline. This is extremely curious; but why should 
brown paper absorb more rays than white paper? 

Mrs. B. I am far from being able to give a satisfactory 
answer to that question. We can form but mere conjecture 
on this point; and suppose that the tendency to absorb, or 
reflect rays, depends on the arrangement of the minute par¬ 
ticles of the body, and that this diversity of arrangement 
renders some bodies susceptible of reflecting one coloured 
ray, and absorbing the others; whilst other bodies have a 



214 ON REFRACTION AND COLOURS. 

tendency (o reflect all the colours, and others again, to ab¬ 
sorb them all. 

Emily. And how do you know which colours bodies 
have a tendency to reflect; or which to absorb? 

Mrs. B. Because a body always appears to be of the 
colour which it reflects; for, as we see only by reflected 
rays, it can not appear but of the colour of those rays. 

Caroline. But we see all bodies of their own natural 
colour, Mrs. B.; the grass and trees, green; the sky, blue; 
the flowers of various hues. 

Mrs. B. True; but why is the grass green?—because it 
absorbs all except the green rays; it is therefore these only 
which the grass and trees reflect to our eyes, and which 
makes them appear green. The sky and flowers, in the 
same manner, reflect the various colours of which they ap¬ 
pear to us; the rose, the red rays; the violet, the blue; the 
jonquil, the yellow, &c. 

Caroline. But these are the permanent colours of the 
grass and flowers, whether the sun’s rays shine on them or 
not. 

Mrs. B. Whenever you see those colours, the flowers 
must be illumined by some light; and light, from whatever 
source it proceeds, is of the same nature, composed of the 
various coloured rays which paint the grass, the flowers, 
and every coloured object in nature. 

Caroline. But, Mrs. B., the grass is green, and the 
flowers are coloured, whether in the dark, or exposed to 
the light? 

Mrs. B. Why should you think so? 

Caroline. It can not be otherwise. 

Mrs. B. A most philosophical reason indeed! But, as 
I never saw them in the dark, you will allow me to dissent 
from your opinion. 

Caroline. What colour do you suppose them to be, then, 
in the dark? 

Mrs. B. None at all; or black, which is the same thing. 
You can never see objects without light. Light is composed 
of colours; therefore there can be no light without colours; 
and though every object is black, or without colour in the 



ON REFRACTION AND COLOURS. 


215 


dark, it becomes coloured, as soon as it becomes visible. It 
is visible, indeed, but by the coloured rays which it reflects; 
therefore we can see it only when coloured. 

Caroline. All you say seems very true, and I know not 
what to object to it; yet it appears at the same time incredi¬ 
ble! What, Mrs. B., are we all as black as negroes, in the 
dark? you make me shudder at the thought. 

Mrs. B . Your vanity need not be alarmed at the idea, 
as you are certain of never being seen in that state. 

Caroline. That is some consolation, undoubtedly; but 
what a melancholy reflection it is, that all nature which ap¬ 
pears so beautifully diversified with colours should be one 
uniform mass of blackness! 

Mrs. B. Is nature less pleasing for being coloured, as 
well as illumined by the rays of light; and are colours less 
beautiful, for being accidental, rather than essential pro¬ 
perties of bodies? 

Providence appears to have decorated nature with the 
enchanting diversity of colours, which we so much admire, 
for the sole purpose of beautifying the scene, and rendering 
it a source of pleasurable employment: it is an ornament 
which embellishes nature, whenever we behold her. What 
reason is there to regret that she does not wear it when she 
is invisible? 

Emily. I confess, Mrs. B., that I have had my doubts, 
as well as Caroline, though she has spared me the. pains of 
expressing them: but I have just thought of an experiment, 
which, if it succeeds, will, I am sure, satisfy us both. It 
is certain, that we can not see bodies in the dark, to know 
whether they have then any colour. But we may place a 
coloured body in a ray of light, which has been refracted by 
a prism; and if your theory is true, the body, of whatever 
colour it naturally is, must appear of the colour of the ray 
in which it is placed; for since it receives no other coloured 
rays, it can reflect no others. 

" Caroline. Oil! that is an excellent thought, Emily; will 
you stand the test, Mrs. B. 

Mrs. B. I consent: but we must darken the room, and 
admit only the ray which is to be refracted; otherwise, the 


216 ON REFRACTION AND COLOURS'. 

white rays will be reflected on the body under trial, from 
various parts of the room. With what do you choose to 
make the experiment? 

Caroline. This rose: look at it, Mrs. B., and tell me 
whether it is possible to deprive it of its beautiful colour? 

Mrs. B. We shall see.—I expose it first to the red rays, 
and the flower appears of a more brilliant hue; but observe 
the green leaves— 

Caroline. They appear neither red nor green; but of a 
dingy brown with a reddish glow! 

Mrs. B. They can not be green, because they have no 
green rays to reflect; neither are they red, because green 
bodies absorb most of the red rays. But though bodies, 
from the arrangement of their particles, have a tendency to 
absorb some rays, and reflect others, yet it is not natural to 
suppose, that bodies are so perfectly uniform in their ar¬ 
rangement,^ as to reflect only pure rays of one colour, and 
perfectly absorb the others; it is found, on the contrary, that 
a body reflects, in great abundance, the rays which deter¬ 
mine its colour, and the others in a greater or less degree, in 
proportion as they are nearer or further from its own colour, 
in the order of refrangibility. The green leaves of the rose, 
therefore, will reflect a few of the red rays, which blended 
with their natural blackness, give them that brown tinge: 
if they reflected none of the red rays, they would appear 
perfectly black. Now I shall hold the rose in the blue rays— 

Caroline. Oh, Emily, Mrs, B. is right! look at the rose: 
it is no longer red, but ot a dingy blue colour. 

Emily. This is the most wonderful of any thing we have 
yet learnt. But Mrs. B., what is the reason that the green 
leaves are of a brighter blue than the rose? 

Mrs. B. The green leaves reflect both blue and yellow 
rays, which produces a green colour. They are now in a 
coloured ray, which they have a tendency to reflect; they, 
therefore, reflect more of the blue rays than the rose, 
(which naturally absorbs that colour,) and will, of course, 
appear of a brighter blue. 

Emily . Yet, in passing the rose through the different 


ON REFRACTION AND COLOURS. 21 7 

colours of the spectrum, the flower takes them more readily 
than the leaves. 

Mrs. B. Because the flower is of a paler hue. Bodies 
which reflect all the rays are white; those which absorb them 
all are black: between these extremes, the body appears 
lighter or darker, in proportion to the quantity of rays they 
reflect or absorb. This rose is of a pale red; it approaches 
nearer to white than black; it therefore reflects rays more 
abundantly than it absorbs them. 

Emily. But if a rose has so strong a tendency to reflect 
rays, I should have imagined that it would be of a deep red 
colour. 

Mrs. B. I mean to say, that it has a general tendency 
to reflect rays. Pale-coloured bodies reflect all the colour¬ 
ed rays to a certain degree, which produces their paleness, 
approaching to whiteness: but one colour they reflect more 
than the rest: this predominates over the white, and deter¬ 
mines the colour of the body. Since, then, bodies of a pale 
colour in some degree reflect all the rays of light, in passing 
through the various colours of the spectrum, they will re¬ 
flect them all with tolerable brilliancy; but will appear most 
vivid in the ray of their natural colour. The green leaves, 
on the contrary, are of a dark colour, bearing a stronger re¬ 
semblance to black, than to white; they have, therefore a 
greater tendency to absorb, than to reflect rays; and reflect¬ 
ing very few of any but the blue, and yellow rays, they will 
appeaT dingy in passing through the other colours of the 
spectrum. 

Caroline. They must, however, reflect great quantities 
of the green rays, to produce so deep a colour. 

Mrs. B. Deepness or darkness of colour proceeds ra¬ 
ther from a deficiency than an abundance of reflected rays. 
Remember that bodies are, of themselves, black; and if a 
body reflects only a few green rays, it will appear of a dark 
green; it is the brightness and intensity of the colour which 
show that a great quantity of rays are reflected. 

Emily A white body, then; which reflects all the rays, 
will appear equally bright in all the colours of the spectrum, 

19 * 


318 


ON REFRACTION AND COLOURS, 


Mrs. B. Certainly. And this is easily proved by pass¬ 
ing a sheet of white paper through the rays of the spectrum. 

Caroline. What is the reason that blue often appears 
green by candle-light? 

Mrs. B. The light of a candle is not so pure as that of 
the sun: it has a yellowish tinge, and when refracted by the 
prism, the yellow rays predominate; and as blue bodies re¬ 
flect the yellow rays in the next proportion (being next in 
order of refrangibility), the superabundance of yellow rays 
gives to blue bodies a greenish hue. 

Caroline. Candle-light must then give to all bodies a 
yellowish tinge, from the excess of yellow rays; and yet it 
is a common remark, that people of a sallow complexion 
appear fairer or whiter by candle-light. 

Mrs. B. The yellow cast of their complexion is not so 
striking, when every object has a yellow tinge. 

Emily. Pray, why does the sun appear red through a 
fog? 

Mrs. B. It is supposed to be owing to the red rays hav¬ 
ing a greater momentum, which gives them power to tra¬ 
verse so dense an atmosphere. For the same reason, the 
sun generally appears red at rising and setting; as the in¬ 
creased quantity of atmosphere, which the oblique rays must 
traverse, loaded with the mists and vapours which are 
usually formed at those times, prevents the other rays from 
reaching us. 

Caroline. And, pray, why are the skies of a blue colour? 

Mrs. B. You should rather say, the atmosphere; for the 
sky is a very vague term, the meaning of which it would be 
difficult to define philosophically. 

Caroline. But the colour of the atmosphere should be 
white, since all the rays traverse it in their passage to the 
earth. 

Mrs. B. Do not forget that we see none of the rays 
which pass from the sun to the earth, excepting those which 
meet our eyes; and this happens only if we look at the sun, 
and thus intercept the rays, in which case, you know, the 
sun appears white. The atmosphere is a transparent me¬ 
dium, through which the sun’s rays pass freely to the earth; 


ON REFRACTION AND COLOURS. 219 

but when reflected back into the atmosphere, their momen¬ 
tum is considerably diminished; and they have not all of 
them power to traverse it a second time. The momentum 
of the blue rays is least; these, therefore, are the most im¬ 
peded in their return, and are chiefly reflected by the atmos¬ 
phere: this reflection is performed in every possible direc¬ 
tion; so that whenever we look at the atmosphere, some of 
these rays fall upon our eyes; hence we see the air of a blue 
colour. If the atmosphere did not reflect any rays, though 
the objects on the surface of the earth would be illuminated, 
the skies would appear perfectly black. 

Caroline. Oh, how melancholy that would be; and how 
pernicious to the sight, to be constantly viewing bright ob¬ 
jects against a black sky. But what is the reason that bo¬ 
dies often change their colour; as leaves which wither in 
autumn, or a spot of ink which produces an iron-mould on 
linen? 

Mrs. B. It arises from some chemical change, which 
takes place in the internal arrangement of the parts, by 
which they lose their tendency to reflect certain colours, 
and acquire the power of reflecting others. A withered 
leaf thus no longer reflects the blue rays; it appears, there¬ 
fore, yellow, or has a slight tendency to reflect several rays 
which produce a dingy brown colour. 

An ink-spot on linen at first absorbs all the rays; but, ex¬ 
posed to the air, it undergoes a chemical change, and the spot 
partially regains its tendency to reflect colours, but with a 
preference to reflect the yellow rays, and such is the colour 
of the iron-mould. 

Emily. Bodies, then, far from being of the colour which 
they appear to possess, are of that colour w r hich they have 
the greatest aversion to, which they will not incorporate 
with, but reject and drive from them. 

Mrs. B. It certainly is so; though I scarcely dare ven¬ 
ture to advance such au opinion, whilst Caroline is contem¬ 
plating her beautiful rose. 

Caroline. My poor rose! you are not satisfied with de¬ 
priving it of colour, but even make it have an aversion to it; 
and I am unable to contradict you. 


220 


ON REFRACTION AND COLOURS. 


Emily. Since dark bodies absorb more solar rays than 
light ones, the former should sooner be heated if exposed to 
the sun? 

Mrs. B. And they are found by experience to be so. 
Have you never observed a black dress to be warmer than 
a white one? 

Emily. Yes, and a white one more dazzling: the black 
is heated by absorbing the rays, the white dazzling by re¬ 
flecting them. 

Caroline. And this was the reason that the brown paper 
was burnt in the focus of the lens, whilst the white paper 
exhibited the most luminous spot, but did not take fire. 

Mrs . B. It was so. It is now full time to conclude our 
lesson. At our next meeting, I shall give you a description 
«f the eye. 


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CONVERSATION XVII, 


OPTICS. 


ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS. 

F-SCRITTION OF THE EYE.-OF THE IMAGE ON THE RETINA.-EFFRACTION 

OF THE HUMOURS OF THE EYE.-OF THE USE OF SPECTACLES.-OF THE 

SINGLE MICROSCOPE.-OF THE DOUBLE MICROSCOPE.-OF THE SOLAR MI- 

CROSCOPE.-MAGICLANTERN.—REFRACTING TELESCOPE.-REFLECTING 

TELESCOPE. 


MRS. B. 

The body of the eye is of a spherical form: (fig. 1. plate 
XXI.) it has two membranous coverings; the external one, 
a a a , is called the sclerotica; this has a projection in that 
part of the eye which is exposed to view, 6 6, which is call¬ 
ed the cornea, because, when dried, it has nearly the con¬ 
sistence of very fine horn, and is sufficiently transparent 
for the light to obtain free passage through it. 

The second membrane which lines the cornea, and en¬ 
velops the eye, is called the choroid, c c c; this has an open¬ 
ing in front, just beneath the cornea, which forms the pupil, 
d d , through which the rays of light pass into the eye. The 
pupil is surrounded by a coloured border called the iris, 
e e, which, by its muscular motion, always preserves the 
pupil of a circular form, whether it is expanded in the dark, 
or contracted by a strong light. This you will understand 
better by examining fig. 2. 

Emily. I did not know that the pupil was susceptible 
of varying its dimensions. 


222 


OPTICS. 


Mrs. B. The construction of the eve is so admirable, 
that it is capable of adapting itself, more or less, to the 
circumstances in which it is placed. In a faint light the 
pupil dilates so as to receive an additional quantity of rays, 
and in a strong light it contracts, in order to prevent the in¬ 
tensity of the light from injuring the optic nerve. Observe 
Emily’s eyes, as she sits looking towards the windows: her 
pupils appear very small, and the iris large. Now, Emily, 
turn from the light, and cover your eyes with your hand, so 
as entirely to exclude it for a few moments. 

Caroline. How very much the pupils of her eyes are 
now enlarged, and the iris diminished. This is, no doubt, 
the reason why the eyes suffer pain, when from darkness 
they suddenly come into a strong light; for the pupil being 
dilated, a quantity of rays must rush in before it has time to 
contract. 

Emily. And when we go from a strong light into ob¬ 
scurity, we first imagine ourselves in total darkness; for a 
sufficient number of rays can not gain admittance into the 
contracted pupil, to enable us to distinguish objects: but in 
a few minutes it dilates, and we clearly perceive objects 
which were before invisible. 

Mrs. B. It is just so. The choroid c c, is embued with 
a black liquor, which serves to absorb all the rays that are 
irregularly reflected, and to convert the body of the eye into 
a more perfect camera obscura. When the pupil is expand¬ 
ed to its utmost extent, it is capable of admitting ten times 
the quantity of light that it does when most contracted. In 
cats, and animals which are said to see in the dark, the 
power of dilation and contraction of the pupil is still greater; 
it is computed that their pupils may receive one hundred 
times more light at one time than at another. 

Within these coverings of the eye-ball are contained three 
transparent substances called humours. The first occupies 
the space immediately behind the cornea, and is called the 
aqueous humour, //, from its liquidity and its resemblance 
to water. Beyond this is situated the crystalline humour, 
gg, so called from its clearness and transparency: it has the 
fgrm of a lens, and refracts the rays of light in a greater de- 




OPTICS. 


223 


gree of perfection than any that have been constructed by 
art: it is attached b\ two muscles, m m, to each side of the 
choroid. The back part of the eye, between the crystalline 
humour and the retina, is filled by the vitreous humour, A h , 
which derives its name from a resemblance it is supposed 
to bear to glass or vitrified substances. 

The membranous coverings of the eye are intended chiefly 
for the preservation of the retina, i i , which is by far the 
most important part of the eye, as it is that which receives 
the impression of the objects of sight, and conveys it to the 
mind. The retina consists of an expansion of the optic nerve, 
of a most perfect whiteness: it proceeds from the brain, en¬ 
ters the eye, at n, on the side next the nose, and is finely 
spread over the interior surface of the choroid. 

The rays of light which enter the eye by the pupil are 
refracted by the several humours in their passage through 
them, and unite in a focus on the retina. 

Caroline. I do not understand the use of these refracting 
humours: the image of objects is represented in the camera 
obscura, without any such assistance. 

JMrs. B. That is true; but the representation would be 
much more strong and distinct, if we enlarged the opening 
of the camera obscura, and received the rays into it through 
a lens. 

I have told you that rays proceed from bodies in all pos¬ 
sible directions. We must, therefore, consider every part 
of an object which sends rays to our eyes, as points from 
which the rays diverge, as from a centre. 

Emily. These divergent rays, issuing from a single point, 
I believe you told us, were called a pencil of rays? 

Mrs . B. Yes. Now, divergent rays, on entering the 
pupil, do not cross each other; the pupil, however, is suffi¬ 
ciently large to admit a small pencil of them; and these, if 
not refracted to a focus by the humours, would continue 
diverging after they had passed the pupil, would fall dis¬ 
persed upon the retina, and thus the image of a single point 
would he expanded over a large portion of the retina. The 
divergent rays from every other point of the object would be 
spread over a similar extent of space, and would interfere 


224 


OPTICS. 


and be confounded with the first; so that no distinct image 
could be formed, and the retina would represent total con¬ 
fusion both of figure and colour. Fig. 3. represents two 
pencils of rays issuing from two points of the tree A B, and 
entering the pupil C, refracted by the crystalline humour 
I), and forming distinct images of the spot they proceed 
from, on the retina, at a b. Fig. 4. differs from the pre¬ 
ceding, merely from not being supplied with a lens; in con¬ 
sequence of which the pencils of rays are not refracted to a 
focus, and no distinct image is formed on the retina. I have 
delineated only the rays issuing from two points of an object, 
and distinguished the two pencils in fig. 4. by describing one 
of them with dotted lines: the interference of these two pen¬ 
cils of rays on the retina will enable you to form an idea of 
the confusion which would arise, from thousands and mil¬ 
lions of points at the same instant pouring their divergent 
rays upon the retina. 

Emily . True; but I do not yet well understand how the 
refracting humours remedy this imperfection. 

Mrs. B. The refraction of these several humours unite 
the whole of a pencil of rays, proceeding from any one point 
of an object, to a corresponding point on the retina, and the 
image is thus rendered distinct and strong. If you conceive, 
in fig. 3., every point of the tree to send forth a pencil of 
rays similar to those, A B, every part of the tree will be as 
accurately represented on the retina as the points a b. 

Emily. How admirably, how wonderfully, this is con¬ 
trived! 

Caroline . But since the eye requires refracting humours 
in order to have a distinct representation formed on the re¬ 
tina, why is not the same refraction necessary for the image 
formed in the camera obscura? 

Mrs. B. Because the aperture through which we re¬ 
ceived the rays into the camera obscura is extremely small; 
so that but very few of the rays diverging from a point gain 
admittance; but we will now enlarge the aperture, and fur¬ 
nish it with a lens, and you will find the landscape to be 
more perfectly represented. 





Fl atj- xjoi. 



















































































OPTICS. 


225 


Caroline, llow obscure and confused the image is now 
that you have enlarged the opening, without putting in the 
lens. 

Mrs. B. Such, or very similar, would be the represen¬ 
tation on the retina, unassisted by the refracting humours. 
But see what a difference is produced by the introduction 
of the lens, which collects each pencil of divergent rays into 
their several foci. 

Caroline. The alteration is wonderful: the representa¬ 
tion is more clear, vivid, and beautiful than ever. 

Mrs. B. You will now be able to understand the nature 
of that imperfection of sight, which arises from the eyes be¬ 
ing too prominent. In such cases, the crystalline humour, 
D, (fig. 5.) being extremely convex, refracts the rays too 
much, and collects a pencil, proceeding from the object 
A B, into a focus, F, before they reach the retina. From 
this focus, the rays proceed diverging, and consequently 
form a very confused image on the retina, at a b. This is 
the defect of short sighted people. 

Emily. I understand it perfectly. But why is this de- . 
feet remedied by bringing the object nearer to the eye, as 
we fiud to be the case with short-sighted people? 

Mrs. B The nearer you bring an object to your eye, 
the more divergent the rays fall upon the crystalline humour, 
and they are consequently not so soon converged to a focus: 
this focus, therefore, either falls upon the retina, or at least 
approaches nearer to it, and the object is proportionably dis¬ 
tinct, as in fig. 6. 

Emily. The nearer, then, you bring an object to a lens, 
the further the image recedes behind it. 

Mrs. B. Certainly. But short-sighted persons have 
another resource for objects which they can not approach to ✓ 
their eyes; this is to place a concave lens, C D, (fig. 1. plate 
XXIL) before the eye, in order to increase the divergence of 
the rays. The effect of a concave lens is, you know, ex¬ 
actly the reverse of a convex one: it renders parallel rays 
divergent, and those which are already divergent, still more 
so. By the assistance of such glasses, therefore, the rays 
from a distant object fall on the pupil, as divergent as those 



226 


OPTICS. 


from a less distant object; and, with short-sighted people, 
they throw the image of a distant object back as far as the 
retina. 

Caroline. This is an excellent contrivance, indeed. 

Mrs . B. And tell me, what remedy would you devise 
for such persons as have a contrary defect in their sight; that 
is to say, in whom the crystalline humour, being too flat, 
does not refract the rays sufficiently, so that they reach the 
retina before they are converged to a point? 

Caroline. I suppose that a contrary remedy must be ap¬ 
plied to this defect; that is to say, a convex lens, L M, fig. 
2., to make up for the deficiency of convexity of the crystal¬ 
line humour, 0 P. For the convex lens would bring the 
rays nearer together, so that they would fall either less diver¬ 
gent, or parallel on the crystalline humour; and, by being 
sooner converged to a focus, would fall on the retina. 

Mrs. B. Very well, Caroline. This is the reason why 
elderly people, the humours of whose eyes are decayed by 
age, are under the necessity of using convex spectacles. And 
when deprived of that resource, they hold the object at a dis¬ 
tance from their eyes, as in fig. 4., in order to bring the fo¬ 
cus forwarder. 

Caroline. I have often been surprised, when my grand¬ 
father reads without his spectacles, to see him hold the book 
at a considerable distance from his eyes. But I now under¬ 
stand it; for the more distant the object is from the crystal¬ 
line, the nearer the image will be to it. 

Emily. I comprehend the nature of these two opposite 
defects very well; but I can not now conceive, how any 
sight can be perfect: for if the crystalline humour is of a 
proper degree of convexity, to bring the image of distant ob¬ 
jects to a focus on the retina, it will not represent near ob¬ 
jects distinctly; and if, on the contrary, it is adapted to give 
a clear image of near objects, it will produce a very imper¬ 
fect one of distant objects. 

Mrs. B. Your observation is very good, Emily; and it 
is true, that every person would be subject to one of these 
two defects, if we had it not in our power to increase or 
diminish the convexity of the crystalline humour, and to 


OPTICS. 


22 7 


project it towards, or draw it back from the object, as cir¬ 
cumstances require. In a young well-constructed eye, the 
two muscles to which the crystalline humour is attached 
have so perfect a command over it, that the focus of the 
rays constantly falls on the retina, and an image is formed 
equally distinct both of distant objects and of those which 
are near. 

Caroline . In the eyes of fishes, which are the only eyes 
I have ever seen separate from the head, the cornea does 
not protrude, in that part of the eye which is exposed to 
view. 

Mrs. B. The cornea of the eye of a fish is not more 
convex than the rest of the ball of the eye; but to supply 
this deficiency, their crystalline humour is spherical, and 
refracts the rays so much, that it does not require the as¬ 
sistance of the cornea to bring them to a focus on the re¬ 
tina. 

Emily. Pray, what is the reason that we can not see an 
object distinctly, if we approach it very near to the eye? 

Mrs. B. Because the rays fall on the crystalline humour 
too divergent to be refracted to a focus on the retina; the 
confusion, therefore, arising from viewing an object too near 
the eye, is similar to that which proceeds from a flattened 
crystalline humour; the rays reach the retina before they are 
collected to a focus, (fig. 4.) If it were not for this imper¬ 
fection, we should be able to see and distinguish the parts 
of objects, which are now invisible to us from their minute¬ 
ness; for could we approach them very near the eye, their 
image on the retina would be so much magnified as to ren¬ 
der them visible. 

Emily. And could there be no contrivance to convey 
the rays of objects viewed close to the eye, so that they 
should be refracted to a focus on the retina? 

Mrs . B. The microscope is constructed for this purpose. 
The single microscope (fig. 5.) consists simply of a convex 
lens, commonly called a magnifying glass; in the focus of 
which the object is placed, and through which it is viewed: 
by this means, you are enabled to approach your eye very 
near the object, for the lens A B, by diminishing the diver- 


228 


OPTICS. 


gence of the rays, before they enter the pupil C, makes them 
fall parallel on th* crystalline humour D, by whjcfli they are 
refracted to a focus on the retina, at R R. 

Emily. This is a most admirable invention, and nothing 
can be more simple, for the lens magnifies the object merely 
by allowing us to bring it nearer to the eye. 

Mrs. B. Those lenses, therefore, which have the shortest 
focus will magnify the object most, because they enable us 
to bring the object nearest to the eye. 

Emily. But a lens, that has the shortest focus, is most 
bulging or convex; and the protuberance of the lens will 
prevent the eye from approaching very near to the object. 

Mrs. B. This is remedied by making the lens extremely 
small: it may then be spherical without occupying much 
space, and thus unite the advantages of a short focus, and 
of allowing the eye to approach the object. 

Caroline. We have a microscope at home, which is a 
much more complicated instrument than that you have de- 
scribed. 

Mrs. B. It js a double microscope (fig. 6.), in which, 
you see, not the object A B, but a magnified image of it, a b. 
In this microscope, two lenses are employed, the one, L M, 
for the purpose of magnifying the object, is called tbe object- 
glass;.the other, N O, acts on the principle of the single 
microscope, and is called the eye-glass. 

There is another kind of microscope, called the solar mi¬ 
croscope, w'hich is the most wonderful from its great mag¬ 
nifying power: in this we also view an image formed by 
a lens, not the object itself. As the sun shines, I can 
show you the effect of this microscope; but for this purpose, 
we must close the shutters, and admit only a small portion 
of light, through the hole in the window-shutter, which we 
used for the camera obscura. We shall now place the ob¬ 
ject A B, (plate XXIII. fig. 1.) which is a small insect, be¬ 
fore the lens C D, and nearly at its focus: the image EF, 
will then be represented on the opposite wall in the same 
manner as the landscape was in the camera obscura; with 
this difference, that it will be magnified, instead of being 


Plate :xxnr. 










































optics. 229 

diminished. I shall leave you to account for this, by examin¬ 
ing the figure. 

Emily. I see it at once. The image EF is magnified, 
because it is farther from the lens, than the object A B; while 
the representation of the landscape was diminished, because 
it was nearer the lens, than the landscape was. A lens, 
then, answers the purpose equally well, either for magnify¬ 
ing or diminishing objects? 

Mrs. B. Yes: if you wish to magnify the image, you 
place the object near the focus of the lens; if you wish to 
produce a diminished image, you place the object at a dis¬ 
tance from the lens, in order that the image may be formed 
in, or near the focus. 

Caroline. The magnifying power of this microscope, is 
prodigious: but the indistinctness of the image for want of 
light is a great imperfection. Would it not be clearer, if 
the opening in the shutter were enlarged, so as to admit 
more light. 

Mrs. B. If the whole of the light admitted does not fall 
upon the object, the effect will only be to make the room 
lighter, and the image consequently less distinct. 

Emily. But could you not by means of another lens 
bring a large pencil of rays to a focus on the object, and 
thus concentrate the whole of the light admitted upon it? 

Mrs. B. Very well. We shall enlarge the opening, and 
place the lens X Y (fig. 2.) in it, to converge the rays to a 
focus on the object A B. There is but one thing more want¬ 
ing to complete the solar microscope, which I shall leave to 
Carolme’s sagacity to discover. 

Caroline. Our microscope has a small mirror attached 
to it, upon a moveable joint, which can be so adjusted as 
to receive the sun’s rays, and reflect them upon the object: 
if a similar mirror were placed to reflect light upon the lens, 
would it not be a means of illuminating the object more 
perfectly? 

Mrs. B. You are quite right. P Q fig. 2.) is a small 
mirror, placed on the outside of the window-shutter, which 
receives the incident rays S S, and reflects them on the lens 
X Y. Now that we have completed the apparatus let us ex- 

20 * 


230 


OPTICS. 


amine the mites on this piece of cheese, which I place near 
the focus of the lens. 

Caroline. Oh, how much more distinct the image now 
is, and how wonderfully magnified! The mites on the cheese 
look like a drove of pigs scrambling over rocks. 

Emily. I never saw any thing so curious. Now, an 
immense piece of cheese has fallen: one would imagine it 
an earthquake: some of the poor mites must have been crush¬ 
ed; how fast they run,—they absolutely seem to gallop. 

But this microscope can be used only for transparent ob¬ 
jects; as the light must pass through them to form the image 
on the wall? 

Mrs. B. Very minute objects, such as are viewed in a 
microscope, are generally transparent, but when opaque ob¬ 
jects are to be exhibited, a mirror M N (fig. 3.) is used to 
reflect the light on the side of the object next the wall: the 
image is then formed by light reflected from the object, in¬ 
stead of being transmitted through it. 

Emily. Pray, is not a magic lanthorn constructed on the 
same principles? 

Mrs. B. Yes; with this difference, that the light is sup¬ 
plied by a lamp, instead of the sun. 

The microscope is an excellent invention to enable us to 
see and distinguish objects, which are too small to be visible 
to the naked eye. But there are objects, which, though not 
really small, appear so to us, from their distance, to these 
we can not apply the same remedy; for when a house is so 
far distant, as to be seen under the same angle as a mite 
which is close to us, the effect produced on the retina is the 
same: the angle it subtends is not large enough for it to form 
a distinct image on the retina. 

Emily. Since it is impossible, in this case, to approach 
the object to the eye, can not we by means of a lens bring 
an image of it nearer to us? 

Mrs. B. Yes; but then the object being very distant 
from the focus of the lens, the image would be too small to 
be visible to the naked eye. 

Emily . Then, why not look at the image through ano- 


OPTICS. 


£31 


ther lens, which will act as a microscope, enable us to bring 
the image dose to the eye, and thus render it visible? 

Mrs. B. Very well, Emily; I congratulate you on hav¬ 
ing invented a telescope. In figure 4. the lens C D, forms 
an image E F, of the object A B; and the lens X Y serves 
the purpose of magnifying that image; and this is all that is 
required in a common refracting telescope. 

Emily. But in fig. 4 the image is not inverted on the 
retina, as objects usually are: it should therefore appear to 
us inverted; and that is not the case in the telescopes I have 
looked through. 

Mrs. B When it is necessary to represent the image 
erect, two other lenses are required; by which means a se¬ 
cond image is formed, the reverse of the first, and conse¬ 
quently upright. These additional glasses are used to view 
terrestrial objects; for no inconvenience arises from seeing 
the celestial bodies inverted. 

Emily. The difference between a microscope 'and a 
telescope, seems to he this:—a microscope produces a mag¬ 
nified image, because the object is nearest the lens; and a 
telescope produces a diminished image, because the object 
is furthest from the lens. 

Mrs. B. Your observation applies only to the lens C D, 
or object-glass, which serves to bring an image of the object 
nearer the eye; for the lens X Y, or eye-glass, is, in fact, a 
microscope, as its purpose is to magnify the image. 

When a very great magnifying power is required, tele¬ 
scopes are constructed with concave mirrors, instead of 
lenses. Concave mirrors, you know, produce by reflection, 
an effect similar to that of convex lenses by refraction. In 
reflecting telescopes, therefore, mirrors are used in order to 
bring the image nearer the eye; and a lens or eye-glass the 
same as in the refracting telescope to magnify the image. 

The advantage of the reflecting telescope is, that mirrors 
whose focus is six feet, will magnify as much as lenses of a 
hundred feet. 

Caroline. But I thought it was the eye-glass only which 
magnified the image; and that the other lens served to bring 
a diminished image nearer to the eye. 


232 


OPTICS. 


Mrs. B. The image is diminished in comparison to the 
object, it is true; but it is magnified if you compare it to the 
dimensions of which it would appear without the interven¬ 
tion of any optical instrument; and this magnifying power 
is greater in reflecting than in refracting telescopes. 

We must now bring our observations to a conclusion, for 
I have communicated to you the whole of my very limited 
stock of knowledge of Natural Philosophy. If it will enable 
you to make further progress in that science, my wishes will 
be satisfied; but remember that, in order that the study 
of nature may be productive of happiness, it must lead to 
an entire confidence in the wisdom and goodness of its boun« 
teous Author. 


INDEX 


A. 

Aim, 11, 16, 30, 55,153,184, 206. 
Air-pump, 34, 155. 

A ngle, 49 ‘ 

acute, 50. 

• obtuse,* 50. 

of incidence, 50, 1S1, 196. 
of reflection, 50, 173,181, 

. 195 . 

of vision, 191, 197. 
Aphelion, 84. 

Arctic circle, 103, 112. 
Atmosphere, 116, 145, 153, 165, 
184. 

reflection of, 168, 
colour of, 210. 
refraction of, 204, 
208. 

Attraction, 11, 15, 26,204. 

of cohesion, 16, 37, 
133, 154. 

of gravitation, 20, 34, 
79, 91, 107, 128, 
153. 

Avenue, 192. 

Auditory nerve, 174." 

Axis, 87. 

of motion, 53, 61. 
of the earth, 103, 110. 
of mirrors, 199. 
of a lens, 209. 

B. 

Balloon, 33. 

Barometer, 153. 

Bass, 175. 

Bladder, 155. 

Bodies, 10. 

elastic, 44, 55. 
luminous, 178. 
sonorous, 171. 
fall of, 26, 29, 33, 41. 
opaque, 177, 205. 
transparent, 178, 205. 
Bulk, 17. 


C. 

Camera obscura, 185, 215, 228. 
Capillary tubes, 19. 

Centre, 53. 

of gravity, 53, 56, 59, 61, 
' 128. 

of motion, 53, 61, 129. 
of magnitude, 53, 58.. 
.Centrifugal force, 55,82,106,128. 
Centripetal force, 55, 82. 

Ceres, 94. 

Circle, 48, 105, 107. 

Circular motion, 52, 82. 

Clouds, 145, 

Colours, 25, 201. 

Comets, 95. 

Compression, 46. 

Concord, 275. 

Constellation, 96. 

Convergent ray®*!98, 200, 
Crystals, 12, 

Cylinder, 58. 

D. 

Day, 87, 117. 

Degrees, 49, 105, 111, 194. 

of latitude, 106, 124, 
of longitude, 106, 124, 
Density, 20. 

Diagonal, 52, 

Diameter, 105. 

Diurnal, 88. 

Discords, 174. 

Divergent rays, 198, 

Divisibility, 11, 13. 

E. 

Earth, 20, 79, 94, 99, 102, 

Echo, 173. 

Eclipse, 122, 126, 180. 

Ecliptic, 96, 104. 

Elastic bodies, 44, 45. 

fluids, 16, 31, 132, 153. 
Ellipsis, 84 

Essential properties, 11. 


2S4 


INDEX, 


Exhalations, 13. 

Extension, 11, 12. 

Equator, 104. 

Equinox, 112,113. 

precession 170. 

Eve, 185. 

F. 

Fall of bodies, 25, 29, 33, 41. 
Figure, 11, 12. 

Fluids, 133. 

elastic, 133, 153. 
equilibrium of, 134,158. 
pressure of, 135,148,158. 
Flying, 45. 

Focus, 200. 

of convex mirrors, 200. 
of concave, 201. 
of a lens, 209. 

Force, 36. 

centrifugal, 54, 82, 106, 
128. 

centripetal, 54, 82. 
of projection, 55, 80. 
of gravity, 20, 79, 90, 
116, 153. 

Fountains, 151. 

Friction, 77,152. 

Frigid zone, 105, 111. 

Fulcrum, 69. 

G. 

General properties of bodies, 11. 
Georgium Sidus, 94. 

Glass, 208 

refraction of, 208. 
burning, 213. 

Gold, 139. 

Gravity, 20, 25, 34,37,41, 55, 56. 

H. 

Harmony, 174. 

Heat, 18, 115. 

Hemisphere, 103, 112. 
Hydrometer, 143. 

Hydrostatics, 133. 

I. 

Image on the retina, 186,195. 
Image reversed, 188. 

in plain mirror, 195. 


Image in convex ditto, 198. 
in concave, 198. 

Impenetrability, 11. 

Inclined plane, 60, 74. 

Inertia, 11, 15, 36. 

Juno, 94. 

Jupiter, 94,126. 

L. 

Lake, 149. 

Latitude, 106, 124. 

Lens, 209. 

convex, 209. 
concave, 209. 

Lever, 60. 

first order, 65. 
second ditto, 66. 
third ditto, 67. 

Light, 178. 

pencil of, 178. 
reflected, 181. 
of the moon, 183. 
refraction of, 204. 
absorption of, 213. 

Liquids, 133. 

Longitude, 106, 124. 

Luminous bodies, 177. 

Lunar month, 121. 
eclipse, 123. 

M. 

Machine, 60, 73, 75. 

Magic lanthorn, 230. 

Mars, 94. 

Matter, 11, 44. 

Mechanics, 60. 

Mediums, 178, 204, 211. 

Melody, 176. 

Mercury, planet, 93, 127. 

Mercury, or quicksilver, 158. 

Meridians, 105. 

Mieroscope, 222. 

single, 227. 
double, 228. 
solar, 228. 

Minerals, 13. 

Minutes, 105. 

Monsoons, 167. 

Month, lunar, 121. 

Momentum, 42, 64. 

Moon, 89, 90,95,121, 127, 


INDEXi 


235 


Moon-light, 183. 

Motion, 15, 36, 42, 44. 
uniform, 38. 
perpetual, 39. 
retarded, 39. 
accelerated, 40. 
reflected, 48. 
compound, 51, 
circular, 52, 82. 
axis of, 53, 61. 
centre of, 53, 61, 129. 
diurnal, 87. 

Musical instruments, 174. 

Mirrors, 195. 

reflection of, 196. 
plane or flat, 198. 
convex, 198. 
concave, 198, 200. 
axis of, 199. 
burning, 201. 

N. 

Neap tides, 130. 

Nerves, 186. 

auditory, 174, 187. 
optic, 185,187. 
olfactory, 187. 

Night, 87, 

Nodes, 112,119. 

O. 

Octave, 175. 

Odour, 14. 

Opaque bodies, 177. 

Optics, 177. 

Orbit, 92. 

P. 

Pallas, 94. 

Parabola, 56. 

Parallel lines, 28. 

Pellucid bodies, 178. 

Pencil of rays, 178. 

Pendulum, 109. 

Perihelion, 84. 

Perpendicular lines, 28,48, 114. 

Phases, 122. 

Piston, 161. 

Plane, 104. 

Planets, 86, 90,117. 

Poles, 103. 


Polar star. 111, 125. 

Porosity, 46. 

Powers, mechanical, 60. 
Projection, 55, 80. 

Precession of the equinoxes, 117. 
Pulley, 60, 69. 

Pump, 34, 35. 

sucking or lifting, 161. 
forcing, 162, 164. 

Pupil of the eye, 185. 

R. 

Rain, 145. 

Rainbow, 212. 

Rarity, 17. 

Ray of light, 178. 

of reflection, 181. 
of incidence, 181. 

Rays, intersecting, 185. 

Reaction, 43. 

Receiver, 34. 

Reflection of light, 181. 

angle of, 50, 196. 
of mirrors, 196. 
of plain mirrors, 198, 
of convexmirrors,198. 
of concave mirrors, 
198. 

Reflected motion, 48. 

Refraction, 204. 

of the atmosphere, 
207. 

of glass, 208. 
of a lens, 209. 
of a prism, 210. 
Resistance, 60. 

Retina, 185. 

image on, 185. 

Rivers, 144. 

Rivulets, 147. 

S. 

Satellites, 90,119, 126. 

Saturn, 95. 

Scales or balance, 61. 

Screw, 60, 75. 

Shadow, 122,179. 

Siderial time, 118. 

Sight, 185. 

Signs, Zodiac, 96,104, 106. 
Smoke, 14, 32. 


236 INDEX. 


Solar microscope, 228. 
Solstice, 111. 

Sound, 169. 

acute, 174. 
musical, 175. 

Space, 37 

Specific gravity, 138. 

of air, 157. 

Spectrum, 211. 

Speaking trumpet, 173. 
Sphere, 28, 58, 1D7. 
Springs, 144. 

Spring tides, 129. 

Square, 91, 95. 

Stars, 86, 96, 118, 125. 
Storms, 165. 

Substance, 10. 

Summer, 85, 111. 

Sun, 79, 91, 178, 207. 
Swimming, 45. 

Syphon, 148. 

T. 

Tangent, 55, 82. 

Telescope, 231. 

reflecting, 231. 
refracting, 231. 
Temperate zone, 105, 112. 
Thermometer, 160. 

Tides, 127. 

neap, 130. 
spring, 130. 
serial, 168. 

Time, 117,120. 

siderial, 118. 
equal, 120. 
solar, 120.. 

Tone, 176. 

Torrid zone, 104, 112,165. 
Transparent bodies, 178. 
Treble and bass, 175, 
Tropics, 103. 


V. 

Valve, 161. 

Vapour, 19, 33, 145. 

Velocity, 37, 62. 

Venus, 93,127. 

Vesta, 94. 

Vibration, 171. 

Vision, 191. 

angle of; 191. 
double, 195. 

U. 

Undulations, 172. 

Unison, 175. 

W. 

Waters, 133, 146. 

spring, 146. 
rain, 146. 

level of, 134,139, 145. 
Wedge, 60, 74. 

Weight, 17, 25, 108, 139, 154, 
155. 

Wheel and axle, 60, 72. 

Wind, 164. 

trade, 165. 
periodical, 167. 

Winch, 75. 

Winter, 85, 112. 

Y. 

Year, 117. 

siderial, 118. 
solar, 118. 

Z. 

Zodiac, 96. 

Zone, 104. 

torrid, 104, 112, 165, 207. 
temperate, 105, 112. 
frigid, 105, 111. 


THE END. 


QUESTIONS 

ON NATURAL PHILOSOPHY. 

ADAPTED to 

64 CONVERSATIONS ON NATURAL PHILOSOPHY.’* 


CONVERSATION I. 

GENERAL PROPERTIES OF BODIES. 

1. What is meant by the philosophical term body or bodies ? 

2. And what is meant by the term matter ? 

3. What are the essential properties of bodies ? 

4. What is meant by impenetrability ? 

5. What experiment will show, that this property belongs to 
liquids ? 

6 What experiment will show, that it belongs to air ? 

7. What is meant by the extension of a body ? 

8. What is meant by figure ? 

9. What forms has nature assigned to most of her productions ? 
JO. What is said of the natural forms of mineral substances ? 

11. What is meant by divisibility ? 

12. What affords a striking example of this property ? 

13. Is it possible to destroy or annihilate matter ? 

14. What then becomes of that part of coal or wood, which, 
in burning, Is not reduced to ashes? 

15. What is meant by inertia ? 

16. What is the last property common to all bodies? 

17. Of what do all bodies consist ? 

- 18. What is that power called, which unites these together, 

and preserves them in a solid mass ? 

19. Does the attraction of cohesion exist in liquids ? 

20. And do elastick fluids, such as air, possess this property ? 

21. What is meant by the term density ? 22. What by rarity ? 

23. When ^he particles of a body are so near as to attract 
each other, does not their density gradually augment ? 

24. What influence has heat on the particles of bodies ? 

25. And what is the effect of this struggle between the con¬ 
tending forces of heat and attraction ? 

26. On what is the agency of heat the most sensible ? 

27. In what state does rain fall from the clouds ? 

28. What then collects it into drops ? 

29. What other curious effect of the attraction of cohesion is 
there ? 

30. How do you account for it, that liquids thus ascend ? 

31. What substances absorb liquids on this principle ? 




2 


32. What is tiie difference between the attraction of cohesion 
and that of gravitation ? 

33. What other difference is there between the attraction of 
particles and that of masses ? 

34. What instance illustrates this ? 

35. What may further be observed in regard to all solid bodies ? 

36. Since the power of cohesive attraction is so great, why are 
not such bodies as sand and powder, collected by it into large 
and solid masses ? 


CONVERSATION II. 

ATTRACTION OF GRAVITY. 

1. To what is the attraction of gravity proportional ? 

2. Why is it,that all things have a tendency to fall to the ground ? 

3. Why is not weight a property as essential to matter as at¬ 
traction ? 

4. Since attraction is common to all matter, must it not be 
mutual between two bodies ? 

5 Why then does not the earth rise part way to meet the 
stone, which is falling towards it ? 

6. Why does not a plumb-Jine, when suspended on the declivi¬ 
ty of a mountain, fall perpendicularly to the earth ? 

7. Are two lines, which are perpendicular to the earth, paral¬ 
lel to each other ? 

8. What does geometry teach in regard to this ? 

9. What is the law of attraction in regard to the falling of 
bodies at equal distances from the earth ? 

10. But what prevents this law from taking effect ? 

11. To what is the resistance, which the air opposes to the 
fall of bodies, proportional ? 

12. By what means may the heaviest bodies be made to float 
awhile in the air ? 

13. Is the air subject to the laws of gravity ? 

14. Why then does it not, like all other bodies, fall to the 
ground ? 

15. What is an elastick fluid ? 

16. In what region of the atmosphere is the air the most dense ? 

17. What is the principal cause of this ? 

18. Why is it, that such bodies as smoke and steam ascend, 
rather than fall towards the earth ? 

19. What familiar illustration is there of this principle? 

20. How far do vapours rise in the atmosphere ? 

21. Of what is smoke composed, and what is the principal 
cause of its ascent ? 



3 


22. By what means do balloons ascend? 

23. Explain how the gravity of bodies is modified by the ef~ 
■feet of the air ? 

24. What is the use of the air pump ? 


CONVERSATION III. 

LAWS OF MOTION. 

1. tVhat is meant by motion ? 

2. W 7 hat is the power, which puts a body in motion, called ? 

3. In what direction is the motion of a body, acted upon by a 
single force ? 

4. What is meant by the term velocity ? 

5. To what is the velocity of a moving body proportional ? 

6. When is velocity called absolute ? 7. When relative ? 

3. What is the rule for estimating the velocity of a moving body ? 

9. What is meant by the term uniform,when applied to motion : 

10. How is uniform motion produced? 

11. But does not the velocity of bodies, thus put in motion, 
gradually diminish ? 

12. What then is the propriety of terming it uniform ? 

13. What is retarded motion ? 14. What accelerated motion ? 

15. In what manner is the velocity of a falling body accele¬ 
rated by the force of gravity ? 

16. What distance do heavy bodies, descending from a height 
by the force of gravity, fall the first second of time ? what the 
second ? and third ? 

17. To w hat must the impulse of projection, in throwing ^ 
body upwards, be equal ? 

18. What makes these two forces balance each other ? 

19. What is meant by the momentum of a body ? 

20. Of what is it composed ? 

21. If the weight of a body be represented by 3, and its velo¬ 
city by 3, what will be its momentum ? 

22. If a body in motion strike against another body, to what 
w T ill the resistance of the body at rest be equal? 

23. And what is the philosophical language in regard to this 
fact ? 

24. What illustration does figure 3, plate i, afford ? 

25. W T hat is the fact in regard to the six ivory balls, as in 
figure 4 ? 

26. To what class of bodies is mutual action and re-action 
confined ? 

27. In what instances in nature do we observe the usefulness 
of the principle of re-action ? 

28. What does elasticity imply ? 



4 


29. And ou what does this depend ? 

SO. What does the celebrated experiment at Florence show : 

31. What is Sir I. Flew ton’s conjecture in regard to the porous 
nature of bodies ? 

32. What is reflected motion ? 33. What produces it ? 

34. What is meant by a perpendicular line ? 

35. W hat is an angle ? 

36. Illustrate the definition by figure 1, plate ii. 

37. By what is an angle measured ? 

33. Into how many degrees are all circles divided? 

39. When are two angles said to be equal ? 

40. How many degrees are contained in the two angles form¬ 
ed by one line falling perpendicularly on another ? 

41. What forms a right angle, and how many degrees does it 
contain ? 

42. What is an angle, containing more than 90 degrees, called l 

43. And if it contain less, what is it called ? 

44. If a ball is thrown obliquely against the w r all, in what di¬ 
rection will it rebound ? illustrate this by figure 4, plate ii. 

45. What is the first angle called? what the second ? and how 
does one compare with the other ? 


CONVERSATION IV. 

COMPOUND MOTION. 

1. What will be the effect, if a body be struck by two equal 
forces in opposite directions ? 

2. What will be the effect, if the forces strike the body in two 
.directions inclined to each other at an angle of 90 degrees ? 

3. But in what time will it reach D ? (see figure 5, plate ii.) 

4. What is this oblique line called, which the body describes ? 

5. What will be the effect if the forces are unequal ? (see 
figure 6.) 

6. Supposes the forces are unequal, and act in the direction of 
an acufe angle, what will be the effect ? (see figure 7.) 

7. By what means is circular motion produced ? 

3. How js this effected by two forces ? 

9. What instance illustrates this ? 

10. What is meant by the axis of motion ? 

11. Is the centre of motion always in the middle of a body ? 

12. What is the middle point of a body called ? 

13. What one circumstance is there in circular motion, which 
requires careful attention ? 

14. What illustration does figure 1, plate iii, afford ? 

15. What is the force called, which confines a body to the 
Centre, round which it moves ? 



1G. And what is that called, which impels it to Gy lrona the 
centre ? 

17. What would be the effect, were the centripetal force de¬ 
stroyed, and the other force alone to impel the body ? 

18. And what is this line called, which is thus described ? 

19. How many forces act on a ball, which is thrown in a 
horizontal direction ? 

20. What are they ? 

21. How is it that bodies, thus projected, describe a curve-line ? 

22. What is this curve line called in geometry ! 

23. What is meant by the centre of gravity ? 

24. What will be the effect, if any other point of the body is 
alone supported ? 

25. What illustration does figure 4, plate iii, afford ? 

26. By what means does the rope-dancer perform his feats ? 

27. Why do spherical bodies roll down a slope ? 

28. But cannot a cylinder of wood be made to roll up a slope ? 

29. When do the centre of gravity and centre of magnitude 
coincide ? 

30. But when these two centres do not coincide, from what does 
it proceed ? 

31. When two bodies of equal weight are fastened together, 
where will the centre of gravity be ? 

32. But if one body be heavier than the other, where shall we 
find the centre of gravity ? 


CONVERSATION V. 

MECHANICAL POWERS. 

1. What is the number of mechanical powers ? 

2. Can you mention them ? 

3. What four things must be considered, in order to undersand 
the power of a machine ? 

4. What is the lever? 

5. Why are the two scales in an equilibrium, as seen in figure 
I, plate iv ? 

6. How can bodies of unequal weight be made to balance each 

other ? 

7 What are the parts of the lever, divided by the fulcrum, 
called » 

8. What is the relative velocity of a body in regard to its dis¬ 
tance from the axis of motion ? 

9. What does a lever, in its motion, describe ? 

10. On what does the advantage of the effect, produced by the 
lever, depend i 

I* 



0 


11. How many kinds of levers are there it 12. What is the first. 

13. In levers of this kind, what must be the power, in order to 
move the weight, when the fulcrum is equally between both i 

14. What assistance does the lever render in this case ? 

15. In case the fulcrum is nearer the weight than the power, 
and the power less than the weight, how is its deficiency com¬ 
pensated ? 

16. What instrument is there in common use, composed of two 
levers united in one common fulcrum ? 

17. To what is the advantage gained by the second kind of 
levers proportional ? 

18. What is the fact in regard to levers of the third kind, 
Where the power is between the weight and the fulcrum ? 

19. When are levers of this kind used ? 


CONVERSATION V. continued. 

1. What is the pulley ? 

2. What must the power be, compared with the weight, ifr 
order to move it i 

3. What is the advantage of a moveable pulley ? 

4. On what principle are all mechanical powers founded ? 

5. What then is the advantage of the mechanical powers, if 
what we gain by them one way is lost in another ? 

6. Why is the fixed pulley ever used, since it affords us no 
mechanical aid ? 

7. What is the third mechanical power ? 

8. In what proportion is the power increased, in the use of the 
wheel and axle ? 

9. What has been found the most powerful and convenient 
mode of turning the wheel ? 

10. What is one of the great benefits resulting from the use of 
machinery ? 

11. What is the fourth mechanical power ? 

12. How much is the resistance of the weight diminished by the 
use of this power ? 

13. Of what is the wedge composed ? 

14. And what is the advantage gained by it ? 

15. What instruments are constructed on the principle of the 
inclined plane and wedge 1 

16. Of what is the screw composed ? 

17. And what is the construction of the screw and nut ? 

18. To which of the most simple mechanical powers can this 
be referred ? 

19. What is the power of the screw with the addition of the 

lever ? 




20. Wiiat is meant by friction ? a 

21. How much of the power of a machine is reckoned to be 
destroyed by friction ? 

22. What is commonly used to diminish friction ? 

23. What are the two kinds of friction mentioned ? 

24. What is the most considerable ? 

25. What else affects the power of machines ? 

26. What is meant by a medium ? 

27. To what is their resistance proportioned ? 


CONVERSATION VI. 

CAUSES OF THE EARTH*S ANNUAL MOTION. 

1. If the earth at its creation had been projected forward into 
universal space, what would have been its course ? 

2. What does figure 1, plate vi, illustrate? 

3. Into what direction does a ball move, which is struck by 
two forces, acting perpendicular to each other ? 

4 But what produces, in the earth’s motion, an incessant de¬ 
viation from a direct course in a right-line ? 

5. And what force is this attraction of the sun called ? 

6. And what is the impulse of projection called ? 

7. What simple mode is there of illustrating the effect of these 
combined forces ? 

8. Is the motion, in fact, circular, which these two forces pro¬ 
duce ? 

9. What does figure 3, illustrate in regard to this ? 

10. What is the figure called, which the earth describes in its 
course round the sun ? 

11. And what is the place called, which is occupied by the sun ? 

12. Is the earth’s motion as it travels round the sun uniform ? 

13. What can be demonstrated mathematically in regard to a 
body, which moves round a point, towards which it is attracted ? 

14. What illustration of this principle does figure 4, afford ? 

15. What is that part of the earth’s orbit, which is nearest the 
sun, called ? 

16. What is that part called, which is the most distant from 
the sun ? 

17. How much nearer is the earth to the sun, when at its peri¬ 
helion, than at its aphelion? 

18. Is not this the cause of summer and winter ? 

19. What is the difference between the earth’s performing the 
summer half of its orbit and the winter half? 

20. What is the cause of the revolution of the planets round 
the sun ? 

21. But what are the planets ? 



8 


22. What has led to the supposition, that they are inhabitecfr 

23. What are fixed stars? 

24. Why then are not their planets visible to us ? 

25. What makes the planets shine? 

26. Have the planets any other motion besides their revolution 
round the sun ? 

27. What is the axis of a planet ? 

28. Of what is this revolution the cause ? 

29. In what time does the earth perform this revolution ? 

30. What is this motion of the earth called ? 

31. What does this revolution of the earth from west to east 
produce ? 

32. What moral instruction ought we to derive from the con¬ 
sideration of these facts ? 


CONVERSATION VII. 

THE PLANETS. 

I. What are primary planets ? 2. What are secondary planets ? 

3. By what other names are they known ? 

4. To which of these classes belongs our moon ? 

5. In what proportion is the power of attraction diminished by 
distance ? 

6. What is meant by the term square ? 

7. If a planet be twice the distance from the sun, that the 
earth is, how much less would it gravitate? 

8. Why do not the secondary planets move round the sun, 
rather than round their primary planets ? 

9. Round what point do the earth and moon revolve ? 

10. And where is this? 

II. What evidence is there, that the sun revolves on its axis ? 

12. What planet is nearest the sun ? 

13. In what time does Mercury perform his revolution round 
the sun ? 

14. What is the time of his rotation on his axis ? 

15. What is his distance from the sun? 

16. What effect would the heat, to which this planet is ex¬ 
posed, have on metals and on water ?* 

* This calculation is founded on an opinion, once prevalent, 
that the sun is a globe of fire. The best philosophers have re¬ 
jected this idea, and consider the sun as an opaque habitable 
globe, surrounded with a luminous atmosphere, which, coming in 
contact with the atmosphere of the planets, generates the heat, 
which they respectively enjoy. They say w were the sun a 




9 


17. What planet comes next in order ? 

18. What is the distance of Venus from the sun ? 

19. In what time does she revolve about her axis ? 

20. In what time does she perform her revolution round the sun ? 

21. When seen by us before sun-rise, what is she called ? 

22. When seen in the horizon after sun-set, what is she called ? 

23. What planet comes next in order ? 

24. What is the earth’s distance from the sun ? 

25. And what is the term of our annual revolution? 

26. What planet next follows ? 

27. What is Mars’ distance from the sun ? 

28. In what time does he revolve round his axis ? 

29. And in what time perform his annual revolution ? 

30. What is said of the four small planets, which have been 
i*ecently discovered ? 

31. What is remarked of Jupiter? 

32. What is his distance from the sun ? 

33. What is the period of his annual revolution ? 

34. In what time does he revolve round his axis ? 

35. How much larger is Jupiter than the earth ? 

36. What is his diameter ? 

37. With how many satellites attended ? 

33. What is Saturn’s distance from the sun ? 

39. What is the term of his diurnal rotation ? 

40. What his annual revolution ? 41. What is his diameter ? 

42. What is there peculiar to this planet? 

43. And with what number of satellites is he attended ? 

44. By whom was the Georgium Sidus discovered ? 

45. And what his number of moons ? 

46. Are not comets planets ? 

47. What is remarked in regard to their orbits ? 

48. When in that part of their orbit nearest the sun, what ts 
their heat computed to be ? 

49. What number of comets belong to our system? 

50. What is the number, whose revolutions are known ? 

body of fire, it would be hotter on the tops of the mountains than 
in the valleys ; whereas we often find them covered with ice and 
snow, and it is well ascertained, by means of the air balloon, 
that the higher parts of the atmosphere are extremely cold.— 
The spots on the sun’s disk are supposed to be his opaque body 
appearing through his luminous atmosphere, when any part of it 
is more rare or thinner than usual.” The calculation in regard 
to the great heat of the comet is founded on the same opinion.— 
But if the theory of modern astronomers be correct, Saturn may 
enjoy the same heat that Mercury does, and the comet the same, 
when in its aphelion, as in its perihelion. 



10 


51. What are the constellations ? 

52 What should we do in order to learn their proper situa¬ 
tions in the heavens ? 

53. What are the twelve constellations called, through which 
the earth apparently passes, in its annual revolution ? 

54. What are their names ? 

55. What is meant, when it is said that the sun is in a cer¬ 
tain constellation ? 

56. What is this circle called, in which the sun appears to move ? 

57. What is implied, when it is said that a fixed star is in the 
zodiack ? 

58. What is the cause of the apparent difference of size and 
brilliancy of the stars ? 

59. At what rate does an inhabitant of the latitude of London 
move inconsequence of the earth’s diurnal motion ? 

60. And what is the velocity of the earth’s motion round 
the sun ? 

61. What was the system of Ptolemy ? 

62. When was it discarded ? 

63. Who established the present system ? 

64. Who discovered and illustrated the theory of gravitation ? 


CONVERSATION VIII. 

THE EARTH. 

1. What are the two extremities of the earth’s axis called? 

2. What is the equator ? 3. What is a hemisphere ? 

4. What are the small circles called, which surround the poles, 
as E. F. and G. H. in figure 2, plate viii ? 

5. What are the circles I. K. and L. M. called ? 

6. And what is L. K. called,Which, crossing the equator, di¬ 
vides the earth into two equal parts? 

7. Where is the ecliptick situated? 

8. What is meant by the plane of the earth’s orbit ? 

9. Why is the ecliptick described on the terrestrial globe ? 

10. What are the spaces between the several parallel circles 
on the terrestrial globe called ? 

11. Where js the torrid zone? 

12. Where the temperate zones ? 13. Where the frigid zones ? 

14. What are the lines called, which cut the equator at right 
angles ? 

15. What are the greater circles ? 

16. What the lesser circles? 

17. How are all circles divided ? 

18. How are degrees divided? 



11 


19. What is the diameter of a circle ? 

20. How does the diameter of a sphere compare with its cir- 
cumference ? 

21. How many degrees does a meridian, reaching from one 
pole to the other, contain ? 

22. How many degrees from the equator to the poles ? 

23. Into what is theecliptick divided ? 

24 What is latitude, and how are its degrees measured ? 

25. What is longitude, and how measured? 

26. Are the degrees of latitude and longitude, in all circum¬ 
stances, equal to each other ? 

27. What is the length of a degree of latitude ? 

28. Why are not the degrees of longitude at the equator of 
the same length of the degrees of latitude ? 

29. From what does this protuberance about the equator pro¬ 
ceed ? 

30. But how can the centrifugal force produce such an effect ? 

31. What is a sphere or globe? 

32. What part of the earth’s surface is the most strongly at¬ 
tracted ? 

33. On what part of the earth’s surface do bodies weigh the 

least ? 

34. What besides the spheroidical form of the earth has a ten¬ 
dency to increase this difference of weight. ? 

35. Does experiment confirm this theory ? 

36. Wbat instrument is made use of in experiments of this kind ? 

37. Of what does a pendulum consist ? 

38. What is the cause of the pendulum’s motion. 

39. If the velocity of the pendulum impels it, as far beyond 
the perpendicular as it fell, is it not its motion perpetual ? 

40. And why not? 

41. Where does the pendulum vibrate the fastest ? 

42. By what means can a pendulum at one of the poles and 
at the equator be made to vibrate in equal times ? 

43. What must be the length of a pendulum, which vibrates 
seconds in the latitude of London ? 

44. In what direction is the earth’s axis to the plane of its 
orbit ? 

45. If the ecliptick represent the plane of the earth’s orbit, 
what shows the obliquity of its axis in this orbit? 

46. How many degrees is its obliquity ? 

47. What are the points called, where the ecliptick intersects 
the equator ? 

48. When does the summer solstice take place? 

49. At what season of the year is the north pole inclined to¬ 
wards the sun? 

50. What is therefore the consequence of this inclination ? 


12 


51. At this season of the year, what do the inhabitants of the 
north frigid zone enjoy ? 

52. What is that point called, at which the ecliptick crosses 
the equator, when the earth is passing from the summer solstice ? 

53. What may be remarked in regard to the length of the 
days and nights, when the earth is at this point ? 

54. And why is the term equinox applied ? 

55. At what time does the winter solstice take place ? 

56. When does the vernal equinox happen ? 

57. What is the reason, that the sun’s rays afford less heat, 
when in an oblique direction than when perpendicular ? 

58. What illustration does figure 1, plate x, afford ? 

59. What other reason is there, why oblique rays give less 
heat ? 

60. And to what must we refer the diminution of heat, morn¬ 
ing and evening ? 

61. What besides the diminished obliquity of the sun’s rays 
augments the heat of the summer? 

62. What is the reason, that the heat is not the greatest when 
the sun’s rays are the nearest perpendicular? 

63. Have the other planets the same vicissitudes of seasons, 
which the earth has ? 

64. What is the fact in regard to Jupiter ? 

65. What in regard to Mars and Saturn ? 

66. What in regard to Venus ? 

67. What is the reason that there are but 365 days in a year, 
when the earth in this time performs 366 revolutions on her axis ? 

68. If our time were calculated by the stars, instead of the 
sun, would this difference exist ? 

69. What is the reason it would not ? 

70. What is the difference of the apparent revolution of the 
fixed stars and the sun ? 

71. What is the length of the common year and what is it 
called ? 

72. How is it measured ? 

73. But what is the reason, that this year is completed before 
the earth has finished one entire revolution in its orbit ? 

74. What do these variations, caused by the spheroidical figure 
of the earth, produce? 

75. If the equinoctial points, or nodes, are not fixed, what is 
their motion ? 

76. How does figure 1, plate xi, illustrate this? 

77. By what means can we ascertain, when the earth has per¬ 
formed an entire revolution in its orbit ? 

78. What is this year called ? 

79. What is the difference, in regard to time, between the side- 
ral year and the solar ? 


13 


80. What arc the periods, in which the sun and a perfect 
clock agree ? 

81. What is the greatest difference between true time and so¬ 
lar time ? 


CONVERSATION IX. 

THE MOON. 

1. In what time does the moon revolve round the earth? 

2. W hat evidence have we, that the moon turns on her axis ? 

3. What is the length of her day ? 

4. Do all the inhabitants of the moon enjoy the privilege of 
the light, which is reflected from the earth ? 

5. Do they see the earth, under all the changes which the 
moon exhibits to us ? 

6. What are these changes called ? 

7. How does figure 2, plate xi, illustrate these changes ? 

8. When is the moon said to be in conjunction ? 

9. When in opposition ? 

10. What is the cause of the eclipse of the sun ? 

11. What of the moon ? 

12. Why then do we not have a solar and a lunar eclipse 
every month ? 

13. When is an eclipse partial ? 

14. When an eclipse happens precisely at the nodes, what is 
the consequence ? 

15. When the sun is eclipsed, is the darkness total to the 
whole hemisphere ? 

16. And what does this show? 

17. But how is this in regard to lunar eclipses? 

18. What do we discover from the length of time, which the 
moon requires to pass through the earth’s shadow ? 

19. When does the earth appear eclipsed to the inhabitants of 
the moon ? 

20. Are not eclipses frequent in those planets, which are at¬ 
tended with several satellites? 

21. What use has been made of an accurate knowledge of the 
eclipses of Jupiter’s moons? 

22. How is latitude found when at sea ? 

23. Is there any other method ? 

24. From what meridian do the English reckon ? 

25. What does the rotation of the earth on its axis from west 
to east occasion ? 

26. How many degrees does the sun, in his apparent course, 
pass over, every hour ? 

o 




14 


27. if then it be twelve o'clock at Loudon, what will be the 
hour fifteen degrees west ? 

28. By what means can a man at sea make use of this prin¬ 
ciple in ascertaining his longitude ? 

29. Why cannot implicit reliance be placed on this method ? 

30. To what then do they have recourse? 

31. How is it that longitude is ascertained from the eclipses 
of Jupiter's moons ? 

32. What is meant by the transit of a planet ? 

33. What use did astronomers make of the late transit of Ve¬ 
nus ? 

34. What is the cause of the tides ? 

35. But is there not more than one tide in the course of 24 
hours ? 

36. What prevents the earth and moon from meeting at the 
centre of gravity ? 

37. In what proportion does the centrifugal force increase ? 

38. And in what proportion does the power of attraction in¬ 
crease ? 

39. What then is the consequence of these laws iu regard to 
the production of tides ? 

40. Can you illustrate this theory by figure 3, plate xii. 

41. Why does not the sun produce tides ? 

42. When does the sun augment or diminish the tides ? 

43. At what time of the moon do the full tides take place ? 

44. What does figure 5, illustrate ? 

45. Where are the tides the greatest? 

46. Why is it not high water at a place, when the moon is di¬ 
rectly over the meridian of that place ? 

47. How long is it after the moon has passed the meridian be¬ 
fore the effect of her influence is complete ? 

48. Why is the tide three quarters of an hour later every day ? 


CONVERSATION X. 

MECHANICAL PROPERTIES OF FLUIDS. 

1. Of what does hydrostaticks treat? 

2. What is a fluid ? 

3. What is inferred from the slight cohesion, and the facility 
with which particles of fluids slide over each other ? 

4. What is the distinction between a fluid and a liquid ? 

5. Are liquids susceptible of compression ? 

6. To what is this owing ? 

7. What celebrated experiment was made at Florence ? 

8. What is the fact in regard to the gravitation of fluids? 

6., What is meant by the equilibrium of fluids ? 



15 


10. Why is it improper to use the term flat iu this definition ? 

11. Of what is this level of fluids the natural result ? 

12. Of what is a water-level constructed ? 

13. Why do solid bodies gravitate in masses ? 

14. What is the consequence of the particles of fluids acting 
independently of each other ? 

15. From what does the lateral pressure of fluids proceed ? 

16. What does figure 5, plate xiii, illustrate ? 

17. Is the lateral pressure affected by the horizontal dimen¬ 
sions of the vessel, which contains the water ? 

18. In a cubical vessel, how does the pressure downwards 
compare with the lateral pressure ? 

19. From what proceeds the pressure of fluids upwards? 

20. What is the process of this upward pressure, as described 
in figure 4 ? 

21. But if the water be poured into the spout, will it rise in 
the pot to the same height ? 

22. What is the cause of it ? 

23. What is meant by the specific]? gravity of bodies ? 

24. When we say, that lead is heavy, and feathers light, how 
do we speak ? 

25. What is adopted as a standard in estimating the specifick 
gravity of bodies ? 

26. What is the process of the comparison ? 

27. Why does a heavy body weigh less in wafer than out of it ? 

28. To what is the resistance of the fluid proportional ? 

29. How much does a body lose of its weight, when immersed 
in water ? 

30. What is the process of this comparison, when the body is 
lighter than water ? 

31. What will be the consequence, if you place a body in wa¬ 
ter, which has the same specifick gravity, that water has? 

32. How is the specifick gravity of fluids ascertained ? 

33. How is the hydrometer constructed? 


CONVERSATION XI. 

SPRINGS, FOUNTAINS, &C. 

1. What is the cause of the ascent of steam or vapour? 

2. Since vapour is lighter than the air, what prevents its rising 
to the upper regions of the atmosphere ? 

3. What forms clouds? 

4. What is the cause of their falling to the ground in the form 
of rain ? 

5. But since water, in a state of vapour, must be lighter than 
the lower regions of the atmosphere, how can it fall to the earth ? 



6. What are the several changes* which water undergoes in 
its ascent, descent, &c. 

7. What is the difference between rain water and spring water-? 

8. Which is the purest ? 

9. What is the cause of the transparency of spring water? 

10. What is the progress of rain, when it falls on the surface 
of the earth, in the formation of springs, rivulets, &c. 

11. What species of earth will not admit the particles of wa¬ 
ter to pass ? 

12. What illustration does figure 9, afford? 

13. Does a spring ever rise above the level of the reservoir, 
from which it flows ? 

14. On what principle is it, that waiter rises in a duct ? 

15. By what means is the water prevented from rising, in the 
glass called Tantalus’s cup, higher than the breast of the figure ? 

16. Why is it generally so difficult to obtain water by digging 
wells on high land? 

17. But is there not a lake on the summit of mount Cenis, one 
of the Alps ? 

18. Why are wells frequently dry ? 

19. What is the reason, that some springs flow in dry weather 
but are dry in wet weather ? 

20. What is the reason, that rivers usually have their source 
in mountainous countries ? 

21. What is the consequence, if the rivulet runs into a situa¬ 
tion surrounded by higher ground ? 

22. Are not artificial fountains of the nature of springs ? 

23. Why will not the water rise quite as high, as the reservoir r 


CONVERSATION XII. 

MECHANICAL PROPERTIES OF AIR. 

1. What kind of a fluid is the air, or atmosphere ? 

2. Has the attraction of cohesion any influence on elastick 
fluids ? 

3. What is the most essential point, in which air differs from 
other fluids ? 

4. How far does the atmosphere extend above the surface of 
the earth ? 

6. How much weight, is it reckoned, that a man of common 
size sustains, in consequence of the gravity of the atmosphere ? 

6. W hy is he insensible of this ? ‘ 

7. What would he the effect, were a person relieved from 
this external pressure of the atmosphere ? 

8. What does the fact prove, that light bodies in falling to the 
earth are retarded by the air ? 



17 


9. What bursts the bladder, tied over the receiver, when a 
vacuum is produced? 

10. What experiment proves the expansion of the air ? 

11. What other experiment illustrates this property of the air ? 

12. What is the weight of the air? 

13. How can the weight of a small quantity of air be ascer¬ 
tained ? 

14. Why is the temperature of the room observed in estimat¬ 
ing the weight of the air ? 

15. How can the specifick gravity of the air be known ? 

16. What is a barometer? 17. How is it constructed ? 

18. At what height will the weight of the atmosphere sustain 
the mercury? 

19. According to what does the weight of the atmosphere 
vary ? 

20. When is the air the heaviest, in wet weather or dry ? 

21. Why then does the air feel so heavy in unpleasant weather ? 

22. How can the height of mountains be ascertained by the 
barometer ? 

23. What occasions the rise and fall of the fluid in the ther¬ 
mometer ? 

24. When are two fluids of a different density said to be in an 
equilibrium ? 

25. How high a column of water will the weight of the at¬ 
mosphere sustain ? 

26. What instrument, in common use, is constructed on this 
principle ? 

27. What is done by the act of pumping ? 

28. Of what does the pump consist ? 

29. Mention its various parts, as delineated in figure 4, plate 
xiv. 

30. Is the power of suction and that by which water is raised 
in a pump, the same ? 

31. Can water be raised in a pump more than 32 feet ? 

32. Of what does the forcing pump consist ? 

33. Explain it as exhibited in the plate. 


CONVERSATION XIII. 

WIND AND SOUND. 

1. What is wind ? 2. By what is it produced ? 

3. What is the weather in the torrid zone, where the heat is 
the greatest ? 

4. What is the cause of a regular east wind about the equator? 

5. What produces the trade winds? 

2 * 


18 


6. How far on each side of the equator do these winds extend r 

7. If the air is constantly flowing from the poles to the torrid 
zone, why is there not a deficiency of air in the polar regions ? 

8. What example of this circulation can you give on a small 
scale ? 

9. Why are not the periodical winds as regular on land, as at 
sea ? 

10. What are the monsoons ? 

11. How is their variation produced ? 

12. What is meant by their breaking up ? 

13. What occasions the great variety of winds, which occur 
in the temperate zones ? 

14. Why is there usually on a summer’s evening a gentle 
breeze from the ocean ? 

15. Why does the wind usually fall about sunset? 

J6. Why do not the aerial tides affect the barometer? 

17. From what does sound result? 

18. But what is sound, strictly speaking ? 

19. What experiment satisfactorily proves, that sound does 
not exist in sonorous bodies ? 

20. Why cannot the bell be heard in an exhausted receiver ? 

21. What besides the air can convey the vibratory motion of 
-onorous bodies ? 

22. What is a sonorous body ? 

23. To what do they owe this property ? 

24. What illustration does figure 6, plate xiv, afford ? 

25. To what is the effect of a sonorous body on the air similar ? 

26. But if the air reverberates how can its motion extend so 
as to convey sound to a distance ? 

27. At what rate is the velocity of sound computed to be ? 

28. What effect has the direction of the wind on the velocity 
of sound? 

29. To what use can the knowledge of the uniform velocity of 
sound be applied ? 

30. How is the sound of an echo produced ? 

31. On what principle are speaking trumpets constructed ? 

32. Illustrate it by figure 7. 

33. Hoav must the vibrations of a sonorous body be performed 
in order to excite the same uniform idea, or produce one musical 
tone? 

34. What produces such sounds as are called discords ? 

35. Oo what does the acuteness or sharpness of a sound de¬ 
pend ? 

36. On what does the duration of vibrations depend ? 

37. From what do those sounds, which we call harmony ov 
concord, arise ? 

38. What produces the harmony called a fifth ? 


19 


CONVERSATION XIV. 

OPTICKS. 

I. What is a luminous body ? 2. What are opaque bodies ? 

3. What are transparent bodies? 

4. What are transparent bodies frequently called ? 

5. And when the rays of light pass through them, what is the 
term made use of? 

6. When light emanates from the sun, how i3 it projected? 

7. How may every point of the luminous body be considered ? 

8. What is a ray of light ? 

9. What a pencil of rays? 

10. To which of the laws of matter is light subject ? 

II. When the rays of light meet with an opaque body, what 
is the effect ? 

12. What does this interruption produce in regard to the body ? 

13. Why then are shadows of different degrees of darkness ? 

14. To what is the darkness proportional, when the shadow 

is produced by the interruption of the rays of light from a single 
luminous body ? . 

15. When the luminous body is larger than the opaque, what 
will the shadow be ? 

16. Why then is not this the case with the shadows of terres¬ 
trial objects which are illuminated by the sun ? 

17. But when the luminous body is less than the opaque, how 
does the shadow increase ? 

18. When a body is illuminated by a number of lights in dif¬ 
ferent directions, what will be the effect ? 

19. What is meant by reflection ? 

20. If a ray fall perpendicularly on an opaque body, how is it 

reflected ? 

21. If it fall obliquely, in what direction is it reflected ? 

22. What is the law in regard to the angle of incidence ? 

23. By what light do we see opaque objects ? 

24. Why then does one side of the house appear to be in the 
sun-shine, and the other in the shade ? 

25. Why is this better illustrated by the moon-light? 

26. Why do terrestrial objects appear so bright and luminous, 
if we see them by reflected light? 

27. What is there in the atmosphere, which has a tendency 
to absorb the rays of light and render them fainter? 

28. On what principle do we see luminous objects ? 

29. What experiment illustrates this principle? 

30. What is this phenomenon called ? 

31. How is this picture produced ? 

32. What in regard to the eye is represented by the aperture 
in the window-shutter? 


20 


33. And to what is the picture on the wail similar l 

34. From which do we receive the sensation of the presence 
of objects, from this image on the retina of the eye, or from the 
objects themselves ? 

35. How may the nerves, which constitute the sense of sight, 
be considered ? 

36. If we only see the image of objects, why do we not see 
them reversed, as in the camera obscura ? 


CONVERSATION XV. 

opticks— continued. 

1. Why do objects at a distance appear smaller than they 
really are ? 

2. Can you illustrate this by figure 1, plate xvii ? 

3. What does the fact, that objects of the same magnitude ap¬ 
pear to be of different dimensions when at different distances, 
prove ? 

* 4. Why are we not aware of the apparent difference of di¬ 
mension of the objects in the same room with us ? 

5. The laws of what art are founded on this principle ? 

6. How is nature represented in sculpture? 

7. How in painting? 8. When is an object invisible? 

9. What must be the velocity of a body in order to render its 
motion perceptible ? 

10. And is this the reason, why the motion of the celestial 
bodies is invisible ? 

11. What must we know in order to judge of the velocity of 
a body in motion ? 

12. In what respects are we liable to be deceived in regard to 
the sense of sight ? 

13. What corrects the errours, to which we are liable, from 
our senses ? 

14. Why do not objects appear double, since an image must 
be formed on the retina of both eyes ? 

15. When the image of an object is seen in a looking-glass, 
why is it not inverted as in the camera obscura, or on the retina 
of the eye ? 

16. Why can a person see his whole figure in a glass, which 
is but half his height? 

17. But were the mirrour much smaller, would not a ray of 
light from the feet reach it, and thus enable the person to see 
himself ? 

18. Why is it, that a person’s image appears the same dis¬ 
tance behind the glass, that he is in front of it ? 




19. On what account is it that a looking gfass reflects the 
rays of light, since it is a transparent body ? 

20. Why then are not mirrours made simply of mercury ? 

21. What kind of mirrours, more perfect than glass, have 
been introduced ? 

22. Why are not all objects, which reflect light, mirrours? 

23. What kind of bodies make the best mirrours ? 

24. How many kinds of mirrours are used in opticks? 

25. What is the fact in regard to the image formed by the 
plain mirrour? 

26. What is the effect of the convex mirrour ? 

27. What of the concave mirrour ? 

28. Of what is the convex mirrour formed ? 

29. When several rays fail upon it, which is perpendicular to it ? 

30. Where will the other rays A. E. (in figure 1, plate xviii,) 
be reflected r 

31. And why ? 

32. And in this case, where shall we see the image ? 

33. From what two places is this point equally distant; and 

•what is it called ? , 

34. What is meant by a focus ? 

35. Why is this called an imaginary focus ? 

36. Why do objects appear smaller, when viewed in a convex 
mirrour ? 

37. Can }'ou illustrate this by figure 2 ? 

38. Of what are concave mirrours formed ? 

39. Can you illustrate the peculiar property of this class of 
mirrours by figure 3 > 

40. Where are the parallel ray 7 s reflected? 

41. Is this point the imaginary focus ? 

42. And how does its distance from the surface compare with 
its distance from the centre ? 

43. If the raj's fall convergent, where are they brought to a 

focus ? 

44. And what is the fact in this respect in regard to divergent 

rays ? 

45. What are concave mirrours called in consequence of their 
accumulating heat ? 

46. Do rays, which corno from the sun, meet in the true focus 
of the mirrour ? 

47. Place a burning taper in the focus of a concave mirrour, 
and how will its light be reflected ? 

48. In what cases does the image appear within the concave 
mirrour in the same manner as in the convex ? 

49. And why is the image larger than the object ? 


22 


CONVERSATION XVI. 

REFRACTION AND COLOURS. 

1. What is meant by refraction ? 

2. What is the nature of this effect? 

3. What causes this deviation of the ray ? 

4. In what direction does water attract the ray ? 

5. Can you illustrate this by figure 1, plate xix ? 

6. What is the effect, when the ray passes from a dense into a 
rare medium ? 

7. Does the attraction of the denser medium affect the ray 
before it touches it ? 

8. What does figure 3, represent ? 

9. How will the bottom of a clear stream of water appear in 
comparison with its real depth ? 

10. Do we see the heavenly bodies in their real situations ? 

11. In what time does light pass from the sun to the earth? 

12. What effect does the refraction of the sun’s rays have on 
the length of the day ? 

13. When the two surfaces of the refracting medium are pa¬ 
rallel to each other, what will be the direction of the ray, which 
has passed through it ? 

14. What is a lens? 

15. What is the distance of the focus from the surface of the 
lens ? 

16. What is the property of a lens, which has a convex sur¬ 
face ? 

17. What is the property of that, which has a concave surface ? 

18. What is a plano-convex lens ; and a plano-concave ? 

19. Where is the focus of the plano-convex lens? 

20. What is a prism ? 

21. Are the colours, which we see, formed by the prism ? 

22. Why then do the rays of the sun appear colourless ? 

23. To whom are we indebted for the discoveries respecting 
light and colours ? 

24. What is the order of the colours displajmd by the prism ? 

25. IIow does the prism separate these colours ? 

25. Which of the rays deviate the most from their original 
course ? 

27. Which is the least refrangible? 

28. By what means can these rays be re-united ? 

29. And what will be their appearance? 

30. Who has made some of the most accurate experiments on 
light ? 

31. Which does he reckon primitive colours r 

32. How is the rainbow formed ? 




23 


53. What is the difference in regard to the focus of a lens and 
of a concave mirrour ? 

34. Why does brown paper take fire sooner than white, when 
exposed to the focus of a lens ? 

35. Of what is light composed ? 

36. By what are objects rendered visible ? 

37. What colour have objects in the dark? 

38. Place a coloured body in a ray of light, which has been 
refracted by a prism, and what will be the effect ? 

39. When a green body is placed in a red ray, why does it 
not become a bright red ? 

40. How do those bodies appear, which reflect all the rays of 
light ? 

41. From wdiat does the deepness or darkness of a colour pro¬ 
ceed ? 

42. Why does blue often appear green by candle-light? 

43. Why does the sun appear red through the fog ? 

44. Why does the sky or atmosphere appear blue ? 

45. If the atmosphere did not reflect any rays, how would the 
skies appear ? 

46. Which is the warmest, when exposed to the sun’s ray, a 
black dress or a white one ? 

47. And why ? 


CONVERSATION XVII. 

k : v iXti;; pv £ ii $0 . , v v 0 , Jkl // ^ 

OPTICKS. 

1. What is the external covering of the eye called ? 

2. What is that part of the eye exposed to view, as b. b. in 
the figure, called ? 

3. What is the second membrane called ? 

4. What forms the pupil ? 

5. What is the coloured border, which surrounds the pupil, 
called ? 

6. When is the pupil dilated, and when contracted ? 

7. Of what use is the black liquor, with which the choroid is 
imbued ? 

8. How much more light will the pupil admit when expanded, 
than when contracted? 

9. What place in the eye is occupied by the aqueous humour? 

10. Where is the chrystalline humour situated? 

11. What is its form ? 

12. What place is occupied by the vitreous humour ? 

13. For what are membranous coverings of the eye intended? 

14. Of what does the retina consist ? 


24 


15. Whatis the effect of the several humours of the eye on 
the light which the pupil admits ? 

16. If the light admitted by the pupil were not refracted by 
the humours, what would be the consequence ? 

17. What does figure 3, plate xxi, represent ? 

18. And how does figure 4, differ from it? 

19. W hy, in the camera obscura, is it not necessary, that the 
light should be refracted, as in the eye ? 

20. From what proceeds that defect of the eye, which is pe¬ 
culiar to short-sighted people ? 

21. And why is this defect remedied by bringing the object 
nearer the eye ? 

22. What assistance does a concave lens render ? 

23. From what proceeds the defect of the eye, which most 
old people experience ? 

24. In what manner can the convex lens supply this deficiency ? 

25. How is it possible, that the same eye can see objects, 
which are distant, and those which are near, distinctly i 

26. If an object is placed very near the eye, why is it not 
seen distinctly ? 

27. Is there no instrument, which can refract the rays of ob¬ 
jects, thus placed, to a focus on the retina ? 

28. Of what does the microscope consist ? 

29. What is the construction of the double microscope ? 

30. What is the magnifying power of the solar microscope ? 

31. What is its construction ? 

32. Where must the object be placed if you wish to magnify 
the image ? 

33. And where, if you wish to diminish the image ? . 

34. Of what use is the mirrour in the solar microscope ? 

35. What is the construction of a telescope ? 

36. Why, in viewing terrestrial objects, is it necessary to add 
two more glasses ? 

37. When a very great magnifying power is required, how 
are telescopes constructed ? 

38. What is the advantage of the reflecting telescope ? 


THE. END. 


Boston : Joseph W. Ingraham. 


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